Prep: Line with parchment paper and grease an 8×8-inch square pan (9×9-inch pan works, too). Measure 4 cups rice Chex cereal, into a a large bowl and set aside.
Melt: In a heat-proof bowl, melt 2 cups semi-sweet chocolate chips, 1 cup peanut butter, and 1/2 cup butter. You can melt this in the microwave, stirring every 30 seconds until smooth, or use a double boiler for more control. I prefer using a double boiler set up for this step because I can see everything melting and pull the bowl off the heat before anything burns. Be careful, as it will be very hot once fully melted.
Mix with the Chex: Pour the melted chocolate-peanut butter mixture over the cereal in the large bowl. Mix until all the Chex are completely coated with chocolate.
Chill: Transfer the mixture to the prepared pan and gently spread into an even layer. Place the pan in the freezer for 30-45 minutes, or until the bars are firm and fully chilled.
Add the powdered sugar: Sprinkle about 2 tablespoons powdered sugar on the cold muddy buddy bars. Cut and enjoy!