In a large saucepan, melt the butter over low heat. Once melted, add the mini marshmallows and stir continuously until completely melted and smooth. Remove from heat and stir in the vanilla extract.

Quickly add the Chocolate Kickers cereal to the melted marshmallow mixture, stirring until all the cereal is evenly coated. If using, fold in the chocolate chips.

Grease a 9x13-inch baking dish with cooking spray or butter. Pour the cereal mixture into the dish and press it down evenly with a spatula or your hands (damp hands work best to prevent sticking).

Allow the treats to cool at room temperature for about 30 minutes before cutting them into squares.