Preheat the oven to 350 degrees F and coat two 9-inch-round cake pans with cooking spray; line with parchment paper. Prepare the cake mix as directed; transfer the batter to the pans. Bake and cool as directed.
Combine half of the frosting with the crushed candy. Place 1 cake on a small cardboard circle, domed-side down; spread with the candy frosting. Place the other cake on top, domed-side up.
Round the bottom and top edges of the layer cake with a small serrated knife as shown. Cover the whole cake with a thin layer of the remaining plain frosting.
Microwave the marshmallows with 1/4 cup water in a large bowl until melted. Add the peppermint extract; gradually beat in all but 1/2 cup confectioners' sugar with a mixer until thick and stiff. Beat in the butter.
Remove half of the frosting mixture; knead in a little more confectioners' sugar until smooth. Roll out into a 13-inch circle on a piece of parchment dusted with confectioners' sugar.
Carefully invert the rolled-out frosting onto the cake; peel off the parchment, brushing off the excess sugar. Tuck around the bottom edge of the cake, pressing gently to adhere; trim the excess.
Beat enough gel food coloring into the remaining frosting mixture until bright red. Knead in the remaining confectioners' sugar until smooth.
Roll out the red frosting until 1/4 inch thick. Cut a 7-by-1 3/4-inch wavy stripe with a pointed tip out of paper (for a template, go to foodnetwork.com/caketemplate). Using the template, cut out 7 red stripes of frosting.
Carefully arrange the red stripes around the cake with the tips meeting at the center; press gently to adhere and tuck around the bottom of the cake, then trim any excess. Wrap the cake in cellophane.
Photograph by Andrew Purcell
Courtesy of Food Network Magazine