Pour Rice Krispie cereal into a large mixing bowl. Set aside.
Melt butter in a saucepan over low heat. Once butter is melted, pour in mini marshmallows and stir constantly until marshmallows are completely melted and smooth.
Pour marshmallows over Rice Krispies and stir together until completely mixed together.
Scoop Rice Krispies into a lightly grease 8x8 pan. Press down using a piece of wax or parchment paper until flattened. Let cool completely before proceeding. You can pop them in the fridge for 5 minutes to speed up the cooling.