Line an 8x8-inch baking pan with parchment paper for easy removal.
In a large pot over medium heat, melt the butter. Add the mini marshmallows and unsweetened cocoa powder. Stir continuously until the mixture is smooth and melted.
Stir in the vanilla extract and chocolate chips until they are melted and evenly combined.
Gradually add the Rice Krispies cereal to the pot, stirring until all the cereal is evenly coated. If using, fold in the additional mini marshmallows, leaving them mostly whole but softened.
Using buttered hands or a spatula, press the mixture evenly into the prepared pan. Allow it to cool at room temperature for about 2 hours before cutting into squares and serving.