Butter a 9X13 pan. You can use non-stick spray as well, but I like to use real butter when I make no bake treats. My favorite way to butter a pan is to place my hand in a plastic sandwich bag (like a glove) and put a little bit of butter on the edge where my fingertips are. Spread the butter around the bottom and up the sides of the pan until evenly coated.

Measure the cereal out and place it in a large bowl. In a medium saucepan, heat 6 cups of marshmallows, the chocolate chips, corn syrup and butter over low heat, stirring occasionally until melted. Remove from heat and stir in the vanilla extract.

Add the melted chocolate mixture to the cereal and mix until evenly coated. Let the mixture sit for a minute or two to cool a bit and then stir in the remaining marshmallows. Press the mixture into the prepared pan. Let sit for at least an hour before cutting into bars and serving. Store at room temperature.