Place the Rice Krispie cereal in a large mixing bowl and set it aside.

Melt the butter over low heat in a saucepan. As soon as the butter has melted, add the mini marshmallows. Stir constantly until the marshmallows are entirely melted and smooth.

Pour the melted marshmallow mixture over the Rice Krispies. Stir until the cereal is fully coated.

Sprinkle the Reese's Pieces Candy into the mixture. Fold them in until they're evenly distributed.

Transfer the Rice Krispies mixture to a lightly greased 8x8-inch pan. Using wax or parchment paper, press down on the mixture until it is flat. Allow it to cool completely. If you wish to speed up the cooling process, you may refrigerate the pan for 5 minutes.

Once cooled, cut the treats into squares and enjoy!