Line a 9x13-inch baking dish with parchment paper, leaving overhang for easy removal.
In a large saucepan over medium heat, combine the butter and marshmallows. Stir continuously until melted and smooth.
Remove from heat and immediately add the Reese's Peanut Butter Cups cereal. Stir gently to coat.
Fold in the Reese's Pieces until evenly distributed.
Pour the mixture into the prepared baking dish and press down firmly with a spatula or greased hands.
Let the bars cool at room temperature for about 30 minutes or refrigerate to set faster.
Once set, lift the bars out using the parchment paper and cut into squares or rectangles.
