Line a 9x13 inch pan with foil and spray well with cooking spray. The foil is optional, but it makes it easy to remove the bars to slice.

Place butter and marshmallows in a large microwave-safe bowl. Heat on 50% power in 1-minute increments, stirring after each, until melted and smooth (for about 2-3 minutes in an 1100W microwave). While the mixture is heating, measure the cereal into another bowl.

Working quickly once the marshmallows are melted, stir the vanilla into the marshmallow mixture. Add the cereal and Butterfinger bars and stir until all are combined with the melted marshmallow. Pour into prepared pan. Spray your hands with cooking spray and carefully press to compact. (The mixture is hot; be careful not to burn yourself!) Cool before cutting into squares.