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

Pour the mixture into a greased 9-inch square baking dish lined with parchment paper or buttered foil. Gently press the mixture evenly without compacting too tightly. Spread melted peanut butter morsels evenly over the top.

Place the Reese's Mini Peanut Butter Cups over the peanut butter layer and gently press them in. Let the treats set at room temperature for about an hour. Cut into squares and serve.