Spray 13 x 9-inch baking pan with nonstick cooking spray.
Place marshmallows and butter in large saucepan. Heat over medium heat, stirring frequently, until marshmallows are melted; remove from heat. Stir in peanut butter until combined. Stir in cereal and chopped Butterfinger until coated.
Spread into prepared pan. Cool completely; cut into bars.