In 2 separate liquid measuring cups or small bowls, whisk the flavored gelatin packs in 2 cups of boiling water each. Make sure to stir until the gelatin is all the way dissolved. Pour into the prepared dishes and refrigerate for at least 4 hours or preferably overnight.
6 ounces 1 pack green Jell-O , 6 ounces 1 box red Jell-O , 4 cups boiling water
Once totally solidified, spray a 9x13 baking dish with cooking spray.
Cut each Jell-O dish into about 2-inch cubes and sprinkle evenly, mixing up the colors, in the 9x13 baking dish. Place in the fridge until the next step is done.
In a medium mixing bowl, mix together the unflavored gelatin with 2 cups of hot water; it doesn't need to be boiling. Once dissolved, mix in the sweetened condensed milk and vanilla extract. Let the mixture come to room temperature.
2 cups hot water, 14 ounces can sweetened condensed milk, ½ teaspoon vanilla extract, 2 0.25 packets unflavored gelatin
Once it's no longer warm at all, pour over the colored Jell-O cubes. Cover and refrigerate at least 4 hours or preferably overnight.
Once solidified, cut into squares and serve cold.
Notes
Tips:
I let the sweetened condensed milk mixture cool completely before pouring it over the red and green Jell-O. If it's warm, it melts the colors and they turn brown instead of staying distinct.
I chill the can of sweetened condensed milk while the Jell-O sets. When the cold milk gets mixed into the warm plain gelatin, it speeds up the cooling.
I stir each flavored gelatin for a full 2 minutes so it is totally dissolved before it goes into the fridge.
A dollop of whipped cream on top is a nice extra.
Storage:
Keep it covered in the fridge for up to about a week.