In a large pot, melt butter over medium heat. Add marshmallows and stir until completely smooth. Remove from heat and quickly stir in peanut butter, then add Rice Krispies cereal.

Fold in mini marshmallows. Transfer the mixture into a parchment-lined 9-inch square baking dish, pressing gently with a buttered spatula. Spread melted peanut butter morsels evenly on top.

Press the Reese's mini peanut butter cups into the top of the mixture. Let set for about one hour, then cut into squares. Store in an airtight container and enjoy!