Place the Rice Chex in a large bowl and set it aside.
8 cups Rice Chex cereal
In a small microwave-safe bowl, microwave the chocolate chips, peanut butter and butter in 15-second intervals, stirring after each one, until smooth.
1 cup semi-sweet chocolate chips, ½ cup creamy peanut butter, 2 tablespoons unsalted butter
Pour the chocolate mixture over the cereal and stir gently until every piece is evenly coated.
Toss the coated cereal in the powdered sugar until all of the chocolate is covered. A large bowl with a lid or a large resealable bag works best for this step.
1½ to 2 cups confectioners sugar
Spread the coated cereal on a parchment-lined baking sheet and let it set for 20 minutes.
Transfer to a clean bowl, add the pretzels, M&M's, Reese's, marshmallows and sprinkles, and toss until well mixed.
1 cup extra-small mini pretzels, 1 cup red and green M&M's, 1 cup mini marshmallows, 1 cup mini Reese's peanut butter cups, Sprinkles
Notes
Tips:
I melt the chocolate in 15-second bursts and keep stirring for about a minute after the last one, so the heat from the bowl melts the last few chips without scorching them.
I start with 1½ cups of powdered sugar and add more if I want a whiter, snowier coating.
I add the candy after the powdered sugar so the red and green stay bright. Larger sprinkles work best, since tiny ones fall to the bottom.
Storage:
Store Christmas puppy chow in an airtight container or resealable bag at room temperature for up to 5 days. It doesn't need the fridge. To freeze, freeze the coated cereal before adding the pretzels, candy and marshmallows for up to 2 months. Thaw it on the counter, toss with a little more powdered sugar if it feels sticky, then stir in the mix-ins.