In a large pot, melt butter over medium. Add marshmallows; cook, stirring often, until melted, 3 to 5 minutes. Off heat, stir in white chocolate chips and peppermints. Stir in cereal; press into a greased 9-by-13-inch pan. Cool, then cut out with a candy cane cutter.
In a bowl, microwave your favorite store bought frosting on high until melted, 10 to 15 seconds. Drizzle with frosting and sprinkle with crushed peppermints.