How to Make It
Prepare the Cake: Preheat oven to 350°F. Lightly spray 4 (8-inch) round cake pans with cooking spray. Combine milk, vegetable oil, and eggs in the bowl of an electric stand mixer; beat on medium speed until smooth, about 1 minute. Gradually add both packages of cake mix, and beat until smooth, about 2 minutes. Pour batter evenly into prepared pans (about 1 pound batter per pan). Bake in preheated oven until a pick inserted into center of cakes comes out clean, about 25 minutes. Let cool in pans 10 minutes; remove from pans to wire racks to cool completely, about 30 minutes. Wrap each layer in plastic wrap, and freeze until firm, at least 2 hours or overnight.
Prepare the Frosting: Beat butter with an electric mixer until smooth, about 30 seconds. Gradually add powdered sugar, and beat until smooth, about 2 minutes. Beat in vanilla. Place 1/2 cup of the frosting and yellow food coloring in a small bowl; stir until color is fully incorporated, and reserve for buckle. Add red food coloring to remaining frosting in mixer bowl, and beat until color is fully incorporated. Place red frosting in a large piping bag fitted with a large round tip.
Assemble the cake: Remove cake layers from freezer; let stand at room temperature 20 minutes. Place 1 layer of cake on a revolving cake stand. Pipe about 3/4 cup red frosting onto cake layer; smooth evenly with an offset spatula. Place second cake layer on top of smoothed frosting; spread jelly evenly over cake layer. Place third cake layer on top of jelly; continue with frosting as with first cake layer. Top with final cake layer; pipe a small amount of frosting on top and around sides of cake; smooth into a very thin layer all over cake using a bench scraper. (It is okay to see some of the cake through this layer of frosting, this is called a crumb coat and will make the next layer of frosting smoother.) Chill crumb-coated cake 30 minutes. Remove cake from refrigerator, and pipe remaining red frosting on top and sides of cake; smooth frosting using a bench scraper while slowly turning cake stand.
Create Santa’s belt: Place black icing in a small piping bag fitted with a #44 piping tip. Pipe 1 line around center of cake, and continue to pipe until a thick belt is formed. For the buckle, pipe reserved yellow frosting around edge of cookie. Pipe black frosting to fill center of buckle, and pipe a small line of yellow frosting to create a “prong” for the belt buckle. Pipe a small amount of black frosting on back of cookie, and press onto the belt.
Pipe a thin line of red frosting around base of cake; place mini marshmallows side by side in thin line of frosting. Cut larger marshmallows into thirds; discard center third of each marshmallow. Pipe a small amount of red frosting on cut side of each marshmallow to act as “glue”, and place above and below buckle to create a fur trim appearance of Santa’s jacket.