Use a 6-quart slow cooker. Put the butter, chocolate chips, and peanut butter into the slow cooker. Cover and cook on high for about 45 minutes, stirring every 15 minutes.
Line two baking sheets with parchment paper.
When the mixture has softened and melted, stir in the Corn Chex until the mixture has coated the cereal pieces. Spread the coated cereal onto the parchment-lined baking sheets. Sprinkle with powdered sugar.
Place the baking sheets into the refrigerator for at least 2 hours. Break up the snack mix before serving. Leftovers freeze well.