Flag Cake (From-Scratch with Pudding Frosting!)

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This flag cake comes out every 4th of July at my house, and at this point, my family would notice if it didn’t show up.

American flag cake decorated with strawberries, blueberries and piped cream cheese frosting on a sheet pan surrounded by fresh berries and American flag picks.

I’ve made a lot of flag cakes over the years. What I kept coming back to was the frosting. I wanted something that would hold its shape in the heat, pipe into clean stripes and stars without deflating, and not feel heavy by the time dessert rolled around at the end of the night. A pudding and whipped topping frosting does all of that in a way a traditional cream cheese base doesn’t.

The frosting is what makes this version different. A pudding and whipped topping combination is not something you usually see on a from-scratch cake, and that’s the whole point.

The pudding thickens the frosting just enough to hold its shape for piping, while the whipped topping keeps it airy. It doesn’t taste heavy. It tastes like summer.

If you’re building out your 4th of July dessert spread, my 4th of July Trifle is another crowd-pleaser that can be made ahead. And my 4th of July Desserts roundup has even more red, white, and blue ideas for the whole table.

Why This Recipe Works

A flag cake has specific demands. It needs to serve a crowd, look intentional, and hold up when it’s sitting outside at a backyard party in July. The half sheet pan solves the crowd problem. You get more surface area for the flag design and more slices without the cake feeling thin or sparse.

The pudding frosting is what I keep coming back to. It thickens just enough to pipe cleanly and hold its shape without getting stiff or heavy. Cream cheese frosting softens fast in the heat. This one holds. That’s why the design still looks sharp when you’re finally cutting into it at the end of the night.

key Ingredients

Buttermilk. I use buttermilk here and it’s not negotiable. The acid reacts with the baking powder and tenderizes the crumb in a way regular milk won’t. It’s worth picking up a carton.
Sour cream and vegetable oil. I use both, and they work as a team. The oil keeps the crumb moist even after the cake cools to room temperature. The sour cream adds a quiet richness and helps the butter and sugar cream properly. Both stay in.
Instant vanilla pudding mix. This is the ingredient that makes the frosting work the way it does. The pudding thickens the cold milk into a base that holds its shape without being stiff. You end up with a frosting that spreads smoothly, pipes into clean lines and stars, and doesn’t weigh down the fruit. I’ve also made this with white chocolate and cheesecake flavored instant pudding and both work.
Whipped topping, fully thawed. I move mine from the freezer to the fridge the night before. If it’s still partially frozen when it goes into the frosting, it won’t fold in evenly and you’ll end up with lumps. Completely thawed is the only way this works.
Whole milk, very cold. Take it straight from the fridge when you’re ready to make the frosting. Warm milk won’t let the pudding thicken the way it needs to.
Fresh berries, large and uniformly sized. I shop for the biggest, most evenly sized berries I can find, and it makes a real difference. When the blueberries are all roughly the same size, the rows sit evenly. When the strawberry halves are consistent, the stripes line up cleanly.

How to Make Flag Cake

Make the Cake

Step 1: Prep the pan. Preheat the oven to 350°F. Lightly spray a 12x17x1-inch half sheet pan with nonstick baker’s spray and set it aside.

Step 2: Sift the dry ingredients. Sift together 2 1/2 cups all-purpose flour, 2 teaspoons baking powder, and 1/2 teaspoon salt into a medium bowl. Set aside. Hands sifting flour into a glass bowl over a marble surface.

Step 3: Cream the butter and sugar. In the bowl of a stand mixer fitted with the paddle attachment, beat 1 3/4 cups granulated sugar and 3/4 cup room-temperature unsalted butter on medium-high speed for 4 to 5 minutes, until very light and fluffy. I set a timer for this one. The full creaming time is what gives you a light crumb, don’t cut it short.

Step 4: Add the eggs and vanilla. Reduce the speed to low and add 3 large room-temperature eggs one at a time, followed by 1 tablespoon of vanilla extract. Let each egg fully incorporate before adding the next, and scrape the bowl as needed. The batter may look slightly curdled at this point – that’s normal.

Step 5: Add the sour cream and oil. Add 1/2 cup sour cream and 1/4 cup vegetable oil. Mix on low speed until fully combined. Whipped eggs and sugar mixture in a stand mixer bowl.

Step 6: Alternate the flour and buttermilk. With the mixer on low, add the flour mixture and 1 cup buttermilk in alternating additions – start and end with flour. Add a third of the flour, then half the buttermilk, another third of the flour, the remaining buttermilk, then finish with the last of the flour. Let each addition fully incorporate before adding the next, and scrape the bowl as needed. Finished cake batter in a stand mixer bowl.

Step 7: Bake and cool. Spread the batter into an even layer in the prepared pan. Bake for 16 to 18 minutes – I start checking at 16 every time. The cake is done when the edges are very lightly golden, the center springs back when pressed, and a toothpick comes out with just a few moist crumbs. Let it cool completely before adding frosting. A warm cake will melt everything. Baked sheet cake in a rectangular pan.

Make the Frosting

Step 1: Dissolve the powdered sugar. In a large mixing bowl, beat 1 1/2 cups very cold whole milk and 1/2 cup sifted powdered sugar together on low speed for 15 to 30 seconds, until fully dissolved.

Step 2: Add the pudding and thicken. Add both 3.4-ounce boxes of instant vanilla pudding mix and beat for 1 to 1 1/2 minutes, until the mixture thickens. Set it aside for 2 to 3 minutes to let it fully set before folding in the topping.

Step 3: Fold in the whipped topping. Using a rubber spatula, gently fold in 2 containers of fully thawed whipped topping until the frosting is smooth and combined. Cream cheese frosting being folded together with a spatula in a glass bowl.

Decorate the Flag

Step 4: Frost the cake. Spread about two-thirds of the frosting evenly over the cooled cake. Transfer the remaining frosting to a large piping bag fitted with a large star tip. I use a #16 or #21 – either works well for both the stripes and the stars. Sheet cake evenly frosted with cream cheese frosting in a pan.

Step 5: Place the blueberries. In the upper left corner of the cake, arrange 48 blueberries in 6 rows of 8. Take your time here. A messy blueberry section throws off the whole design. Hand placing blueberries in the corner of a frosted sheet cake to create the flag design.

Step 6: Add the strawberry stripes. Place halved strawberries in rows across the remaining frosted area of the cake. Leave white frosting space between each row – that’s where the white stripes go. Flag cake with blueberries and rows of halved strawberries arranged on frosted sheet cake before piping.

Step 7: Pipe the white stripes and stars. Pipe the frosting in straight lines between the rows of strawberries to create the white stripes. Then pipe small stars between the blueberries – about 16 total. Whether you end up with dots or actual stars depends on the tip, but a star tip gives the cleanest result. Hand piping frosting stripes between strawberry rows and adding star details in the blueberry section of the flag cake.

Step 8: Refrigerate until serving. Keep the decorated cake in the refrigerator until you’re ready to serve.

Tips for the Best Results

Check the cake at 16 minutes. I check at the low end every time because this cake bakes faster than you’d expect. Pull it when the center springs back and the toothpick has a few moist crumbs, not wet batter. Overbaking dries out the crumb.
Rinse and completely dry the berries before decorating. I rinse mine and pat them dry with paper towels before they go on the cake. Any moisture left on the fruit will seep into the frosting as the cake sits and soften it faster than it should.
Shop for large, uniformly sized berries. The sizing makes a bigger difference than you’d think. When the berries are all roughly the same size, the rows line up and the flag actually looks like a flag. Look for the biggest blueberries and the most evenly sized strawberries you can find.
Thaw the whipped topping completely before using. I move it from the freezer to the fridge the night before, every time. If it’s still partially frozen when you fold it in, the frosting will be lumpy and won’t pipe or spread right.
Make it the morning of, decorate close to serving. I bake the cake earlier in the day, let it cool completely, and cover it at room temperature. Then I frost and decorate as close to serving time as I can. The berries release moisture as the cake sits, and the frosting softens over time.

American flag sheet cake with strawberry rows, blueberry star section, and white cream cheese frosting on a baking sheet.

Make-Ahead and Storage

I bake this the morning of every time. Let it cool completely, cover the pan tightly, and leave it at room temperature. Frost and decorate within a few hours of serving.
Once frosted and decorated, it will keep in the refrigerator for up to 2 days. The berries release moisture as the cake sits and the frosting will gradually soften.
Same-day is the sweet spot. I don’t recommend frozen fruit for decorating this one.

Sheet cake slice on blue plate topped with rows of halved strawberries and piped whipped frosting.

Frequently Asked Questions

Can I use a boxed cake mix instead?

I only tested this recipe with the from-scratch cake, so that’s what I recommend. The combination of buttermilk, sour cream, and oil is what gives this cake its texture, and a boxed mix will behave differently. If you go the semi-homemade route, know the texture and bake time will vary from what’s described here.

What piping tip should I use for the stripes and stars?

Use a medium star tip for both. Different brands number their tips differently, but a #16 or #21 size will give you clean, defined stripes and stars. If you only have one tip, use it for both.