Line a 13-in. x 9-in. baking pan with waxed paper; grease the paper.
Prepare and bake cake according to package directions, using prepared pan. Cool for 10 minutes; remove from pan to a wire rack to cool completely. Wrap and freeze overnight.
Cover a 20-in. x 15-in. board with gift wrap or foil. Level cake top.
To make boot, beginning from a short side, cut a 2-in.-wide strips. Cut strip into two pieces, one 5-1/2 in. x 2 in. and one 2-1/2 in. x 2 in. Cut a 10-in. x 2-1/2-in. strip from the large rectangle.
To assemble base of boot, center the 10-in. x 2-1/2-in. piece widthwise on board 5 in. from bottom. Place remaining large rectangle on the right side, forming a backward L. Place the 2-1/2-in. x 2-in. piece under the long strip, forming the heal, and place remaining strip on the other side, forming the sole. Using a serrated knife, round corners of boot toe, sole, heal and top.
For frosting, in a large bowl, beat cream cheese and butter until fluffy. Add confectionersâ sugar, milk and vanilla; beat until smooth. Set aside 1 cup. Spread remaining frosting over top and sides of cake.
Tint reserved frosting desired color. Cut a small hole in the corner of a pastry or plastic bag; insert round tip. Fill bag with tinted frosting.
Outline boot shape along bottom and top edges of cake. Fill in heel and toe on cake top, smoothing with a metal icing spatula. With remaining frosting and Skittles, decorate boot as desired. Yield: 16-20 servings.