Melt the butter in a large pan over low heat.
Stir in the peanut butter until melted and combined.
Add the whole marshmallows and melt, stirring constantly to avoid burning.
Remove from heat and stir in the mini marshmallows.
Mix in the mini Reese's Puffs cereal and Rice Krispies cereal until fully coated.
Gently fold in the mini Reese's cups.
Press the mixture into an 8x8 pan lined with parchment paper. Do not press too hard (you don't want to pack the cereal down, as this will make them hard to eat).
Let cool for about two hours.
Slice into squares and enjoy!