How to Make It
Prepare the Cake: Preheat oven to 350°F. Beat softened butter and granulated sugar with a heavy-duty electric stand mixer on medium speed until fluffy, 3 to 5 minutes. Gradually add egg whites, one-third at a time, beating well after each addition.
Sift together cake flour, baking powder, and salt; gradually add to butter mixture alternately with water, beginning and ending with flour mixture. Stir in vanilla extract. Divide batter evenly among 4 small bowls. Add 1 drop of the soft pink food coloring gel to 1 bowl of batter; stir gently until blended. Add 1 drop of the sky blue food coloring gel to second bowl; stir gently until blended. Add 1 drop of the mint green food coloring gel to third bowl; stir gently until blended. Add 2 drops of the yolk yellow food coloring gel to remaining bowl; stir gently until blended. Pour each bowl of colored batter into each of 4 greased (with vegetable shortening) and floured 8-inch round cake pans. Bake in preheated oven until a wooden pick inserted in center comes out clean, 18 to 22 minutes.
Cool cakes in pans on wire racks 10 minutes. Transfer cakes from pans to wire racks, and cool completely, about 30 minutes.
Spread Vanilla Buttercream Frosting on 1 cake layer. Top with second cake layer, and spread frosting on second cake layer. Top with third cake layer. Using a clean, empty can with the top and bottom removed to create a tube shape, place can in center of third cake layer. Press can down through all 3 cake layers, and remove to create a hollowed-out center. Discard cake center or reserve for another use. Fill inside of cake with desired sprinkles and candies. Spread frosting on third cake layer. Cover with fourth cake layer, and spread frosting on top and sides of cake.
Prepare the Horn: Spread a thin layer of frosting over sugar cone, and sprinkle all over with sprinkles until completely coated. Dry completely, about 1 hour.
Prepare the Ears: Divide fondant in half. Roll out 1 portion of the fondant to 1/4-inch thickness. Using a 2 1/2-inch leaf cutter, cut out 2 “ears.” Tint second portion of fondant with 1 drop of the sky blue food coloring gel. Roll out blue fondant to 1/4-inch thickness. Using a 2-inch leaf cutter, cut out 2 smaller leaf shapes to create the inner part of the unicorn ears. Press blue leaves gently into the center of white leaves. Using a clean small paintbrush, brush blue leaves lightly with vodka, and sprinkle with disco dust. Dry completely, 4 to 8 hours.
Assemble the Unicorn Cake: Place the horn on top of the cake, pressing into frosting to adhere. Place 1 ear on each side of horn slightly behind horn, pressing to adhere. Cut licorice pieces to create eyelids and eyelashes, and press lightly into frosting to adhere. Gently curl rainbow candy strips, and arrange as desired on cake to create a mane. If desired, attach sprinkles to mane with small dots of frosting.