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

In a saucepan over low heat, melt the butter.

Once the butter is melted, add the mini marshmallows. Stir continuously until the marshmallows are completely melted and the mixture is smooth.

Mix in the peanut butter and stir until well combined and smooth. Remove from heat.

Pour the marshmallow mixture over the Rice Krispies and stir until thoroughly combined.

Transfer the mixture to a lightly greased 8x8 pan. Press down firmly using a piece of wax or parchment paper to flatten.

Allow to cool completely. For quicker cooling, refrigerate for about 5 minutes.

Once cooled, cut into squares and serve.