Instead of making a Christmas tree cake in a tree-shaped pan, just create Christmas trees from individual cake wedges.
Prepare spice cake recipe using 2 (9-inch) cake pans.
Omit milk in Vanilla Buttercream recipe and tint icing with Wilton Kelly Green icing color. (Or use store-bought green frosting). Spread ¾ cup frosting between layers. Spread a very thin layer (crumb coat) of frosting over top and sides of cake. Cut cake into 8 wedges using a sharp serrated or electric knife. Arrange wedges on their sides on a sheet pan; freeze 1 hour.
Spread icing over all sides of each cake wedge. Decorate with licorice candy to form garlands and Skittles for ornaments. For star: tint Wilton Ready-to-Use Gum Paste with Golden Yellow icing color. Roll out to 1/3 inch thickness; cut with a small star cookie cutter. Secure stars to top of each cake wedge with a toothpick.
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