Special equipment: 8-inch lollipop stick; a jar filled with dried beans or rice; four 8-inch round cake pans; a cake stand
For the unicorn decorations: First, make the horn and ears. Put the marshmallows in a large microwave-safe bowl and sprinkle with 2 teaspoons water. Microwave in 30-second intervals, stirring in between each, until the marshmallows are melted, about 1 1/2 minutes total. Add 1 1/2 cups of the confectioners' sugar and stir to make a stiff mixture. Grease your hands and a work surface with some shortening. Knead the mixture, adding up to 1/2 cup more confectioners' sugar and greasing your hands as needed to keep it from sticking, until the mixture is very smooth and pliable, 4 to 5 minutes. (The mixture should form a tight but very smooth ball. If you poke a finger into it, the indentation should remain.)
Break off three-quarters of the marshmallow mixture, add 1 drop blue gel food coloring and knead it into the mixture until evenly distributed. Remove one-quarter of the blue mixture and roll out to a thickness of a little less than 1/4 inch. With a sharp paring knife, cut two 2-inch-tall triangles for the inside of the unicorn ears. Roll the remaining blue mixture into a thick, tapered strand, about 2 feet long. Brush an 8-inch lollipop stick with water. Starting about 3 inches up from the bottom of the stick, roll the strand around it, starting with the thickest part first, into a tapered horn. Press gently all around to adhere it to the stick. Stick the horn upright in a jar filled with dried beans or rice to dry.
Roll out the remaining plain marshmallow mixture to a thickness of a little less than 1/4 inch. With a sharp paring knife, cut two 3-inch-tall triangles for the outside of the unicorn ears. Brush the larger ears on one side with a small amount of water. Stick on the smaller blue ears. Drape the ears, with the smaller pieces face down, over the rolling pin to dry.
For the cake: Preheat the oven to 350 degrees F. Coat four 8-inch round cake pans with cooking spray and dust with flour, tapping out the excess. Prepare the cake mixes according to the package directions. Stir in the sprinkles. (If you don't have 4 pans, make 1 box of cake mix at a time and use half of the sprinkles in each).
Divide the batter among the 4 prepared pans and bake, rotating the pans from top to bottom halfway through baking, until a tester inserted in the center comes out clean, 25 to 28 minutes. Let cool on a rack for 10 minutes, then remove from the pans and cool completely.
For the buttercream: Beat the butter and salt in a large bowl with an electric mixer on medium speed until smooth and light, about 1 minute. Add the confectioners' sugar and beat on low to incorporate, then increase the speed to medium high and beat until fluffy, 1 to 2 minutes. Beat in the vanilla and then the milk, 1 tablespoon at a time, until the frosting is easily spreadable.
Cut the domed tops from the cakes to make them flat. Frost the top of one cake layer with about 1/2 cup frosting. Set another layer on top and frost again. Repeat with the third layer, then end with the last layer bottom-side up on top. Coat the cake with a very thin layer of frosting to crumb coat, then chill until set, about 15 minutes. Set the cake on a cake stand and spread all but 1/2 cup of the remaining frosting on the cake in a smooth layer.
Cut two 3-inch strands from the licorice and stick to the cake about halfway down in wide U shapes for the eyes. Cut ten 1/2-inch licorice pieces and press 5 along the bottom of each eye for eyelashes. Wrap the rainbow candy belt around the base of the cake. Spread the remaining 1/2 cup frosting in a strip along the base of the cake to adhere the candies. Stick the candies around the cake in a decorative fashion. Pull the cotton candy apart to make it fluffy and stick on the top and sides of the cake as a mane. Add the horn and ears and serve immediately. (Most of the cake can be decorated in advance; the cotton candy needs to be added at the last minute.)
You can also substitute 4 1/2 cups prepared white frosting for the buttercream.
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