Purchase the Carvel® Ice Cream Cakes and place in the freezer until ready to decorate. You will need to decorate the Unicorn Ice Cream Cake in steps to keep the cake from melting.
Prepare 4 batches of Stabilized Whipped Cream Frosting and color 3 of the frosting batches with your choice of gel food coloring as you make them. (I used teal, electric pink and violet). Leave one batch uncolored (white) to use to cover the unicorn ears and to patch up any of the white cake frosting on the ice cream cake if needed. As you make each individual batch of frosting immediately put it in a 12-inch frosting decorating bag fitted with a coupler and piping tip. Once the bag is filled and closed securely with a rubberband place it in the refrigerator. Repeat with each batch of frosting.
For each batch of Stabilized Whipped Cream frosting, you will need a 12-inch frosting decorator bag, a coupler, and a large open star decorator tip. (See Notes below on options for cake decorating with multiple colors of frostings).
While the colored frosting is chilling in the refrigerator use the white frosting to cover the front of the unicorn ears. Use a small offset spatula or a knife to smooth the frosting on the earpieces. If desired you can use pink sparkling sugar to fill in the center of the unicorn ears. Place both frosted unicorn ears on a plate in the freezer until you plan on inserting them into the Unicorn Ice Cream Cake.
If you have the space in your freezer that will allow for the height of the unicorn horn insert the horn towards the front center edge of the round cake. If you don't have room in the freezer slightly push the horn in where you want it to go to mark the spot before you start piping the frosting flowers. This way you will not pipe flowers in that area because when you do insert the horn it will smash the frosting flowers and possible ruin the front of the unicorn cake.
Begin by piping some flowers along the front of the unicorn head and along the back side bringing them down the front corn of the lower cake. Be sure to leave some space between the flowers so you can pipe the next color frosting. Carefully put the ice cream cake back into the freezer for 30 to 60 minutes to keep it frozen.
After 30 to 60 minutes take the ice cream cake out of the freezer and pipe the second color of whipped cream frosting flowers next to the ones you have already piped. Return the cake to the freezer again for 30 to 60 minutes. Repeat with the third color of frosting until you are happy with the way it looks, filling in any gaps with frosting flowers. Freeze the cake overnight.
Decorate the cake with a border of the pastel color Sixletts or sprinkles if desired.
To serve the Unicorn Ice Cream Cake place the cardboard rectangle cake board on the table where you plan on serving and cutting the cake. Arrange the pastel colored cotton candy around the edge of the cake not touching the cake frosting (the cotton candy will melt). Insert the unicorn horn if you haven't already.
Cut and serve the cake. Return the cake to the freezer once you have served all your guests to prevent the Unicorn Ice Cream Cake from melting.