Christmas Tree Cake

Preheat oven to 350°F with racks in top third and bottom third positions. Coat 2 (8-inch) round cake pans and 3 (6-inch) round cake pans with baking spray. Prepare cake batter according to package directions. Spoon about 2 1/2 cups batter into each 8-inch pan and about 1 1/3 cups batter into each 6-inch pan.

Place all cake pans in preheated oven, evenly spaced on racks and without touching. Bake until a wooden pick inserted in centers of cake layers comes out clean, 24 to 26 minutes for 6-inch pans and 26 to 28 minutes for 8-inch pans. Let cake layers cool in pans on a wire rack 5 minutes. Remove cake layers from pans, and let cool completely, about 30 minutes. Tightly wrap each cake layer in plastic wrap; freeze until firm, about 1 hour.

Meanwhile, beat butter with a stand mixer fitted with a paddle attachment on medium speed until pale and smooth, about 2 minutes. Reduce mixer speed to low; gradually add powdered sugar, beating until fully incorporated and smooth, about 2 minutes, stopping to scrape down sides of bowl as needed. Add vanilla, salt, and food coloring; beat until well combined. Add additional blue or yellow food coloring until desired color. Set aside at room temperature until ready to use.

Remove cake layers from freezer, and unwrap. Using a serrated knife, gently slice and remove the domed top from each cake layer, leaving 1 (6-inch) cake untrimmed (still domed). Place 1 of the 8-inch cake layers on a cake plate. Using an offset spatula, spread top of cake layer with about 1/2 cup frosting, spreading to edges. Top with remaining 8-inch cake layer; repeat frosting layer.

Place 1 of the trimmed 6-inch cake layers in center of cake stack on cake plate; spread top with about 1/3 cup frosting, spreading to edges. Top with remaining trimmed 6-inch cake layer; repeat frosting layer. Top with domed 6-inch cake layer.

Using a serrated knife, carve cake stack into a cone shape, starting at the top center of the top cake layer and cutting downward with the knife pointed outward at a 30-degree angle, carving around all sides of cake stack and leaving bottom layer mostly uncut. Discard cake scraps, or reserve for another use.

Using an offset spatula, spread a smooth, thin layer of frosting (about 1 cup) around all sides of cone-shaped cake. Chill cake, uncovered, until frosting is dry to the touch, about 30 minutes. (Let remaining frosting stand at room temperature until ready to use.)

Spoon remaining frosting into a piping bag fitted with a star tip. Starting at base of cake, pipe a ring of quarter-sized dollops of frosting around cake. Repeat piping directly above first ring. Repeat until entire cake is covered. Decorate cake with chocolate pieces and popcorn. Store cake, loosely covered, in a cool, dark place up to 2 days, or in refrigerator up to 1 week.