How to Make It
Preheat oven to 350°F. Grease and flour 2 (8-inch) round cake pans; set aside.
Combine cake mix, water, oil, and eggs in a large bowl. Beat with an electric mixer on low speed until combined, about 1 minute. Divide batter evenly between prepared pans. Bake in preheated oven until a wooden pick inserted in center comes out clean, 20 to 22 minutes.
Cool cakes in pans on a wire rack 15 minutes. Remove cakes from pans, and cool completely on rack, about 30 minutes.
Combine frosting and powdered sugar in a large bowl. Beat with an electric mixer on medium speed until combined and smooth, about 2 minutes. Place 1/4 cup of the frosting mixture in a small bowl, and place 1 cup of the frosting mixture in a separate bowl. Divide remaining frosting mixture evenly among 3 medium bowls. Add brown food coloring to 1/4 cup frosting mixture to create light brown frosting. Add green food coloring to 1 cup frosting mixture to create green frosting. Color remaining frosting mixture 2 different shades of orange: light orange, medium orange, and deep orange.
Cut an arc in 1 spice cake to create a half-moon shape. Remove smaller piece of cake, and cut a small rectangle from smaller piece. Fit second whole cake into arc, and place small rectangle on top to create pumpkin stem. Place orange frostings in each of 3 piping bags, and decoratively pipe over pumpkin. Spread with a knife to merge shades of orange. Place light brown frosting in a piping bag, and pipe a small amount on base of stem to attach to cake. Pipe light brown frosting to cover stem, and spread with knife to smooth. Place green icing in a separate piping bag, and decoratively pipe vines and leaves over cake. Reserve remaining cake for another use.