Using frozen or very chilled cake layers, use a little buttercream to adhere a layer of cake to a cake board. Using an offset spatula, spread a generous layer of frosting covering the entire cake. Place another layer on top of frosting. Repeat, but turn the top layer of cake upside down before placing on top.
Lightly spread frosting on the entire cake, top and sides in a very thin layer. It’s okay if cake is showing through or there are crumbs. This is the crumb coat.
Place the cake in the freezer and freeze for a minimum of 10 minutes. After removing from freezer, spread a thick layer of frosting, covering the cake. Use a cake scraper (bench scraper) to go around the cake and scrape off the excess frosting. Patch any holes with more buttercream and scrape again. Repeat this process until you are satisfied with the smoothness of the cake.
Decorate as you wish! Immediately after frosting place a large candy cane on the side to look like a cocoa mug handle. Use smaller candy canes to decorate the sides. Continue to decorate with sprinkles, crushed candy canes, etc.
Place the cake in the freezer for a few minutes AFTER decorating the sides BEFORE decorating the top. Then top the cake off with additional decorations (frosting swirls, marshmallows, etc).