Grease and line 2 22 centimeter cake tins. Prepare cake batter according to package instructions, and divide evenly into four bowls.
Add a little bit of purple, pink, and blue gel food colour to three of the bowls, leaving the last one plain. Mix, and add more gel until you achieve your desired shade of each of these colours. The more unicorn-like, the better.
Pour 3/4 of the purple batter in the middle of the first pan. Pour 2/3 of the blue batter in the middle of the pan. Pour 1/3 of the white batter in the middle of the pan. Pour 1/4 of the pink batter in the middle of the pan. The batters will spread out as you layer them, creating distinct circles of colour.
Into the second pan, pour 3/4 of the pink batter into the middle of the pan, followed by 2/3 of white batter, 1/3 of the blue batter, and 1/4 of the purple batter.
Bake the cakes according to package instructions. Unmold, and allow to completely cool.
To prepare buttercream, whisk together butter, vanilla and powdered sugar until fluffy. Slowly add cream, until the mixture is thick, smooth, and spreadable.
Trim sugar cones so that they are approximately eight centimeters in height. Coat cones in buttercream, and roll in rainbow sprinkles. Place in the freezer for five to ten minutes to set.
Spread buttercream on one cake layer, and top with the second. Completely cover both cakes in buttercream, and spread to create a smooth surface.
Place sugar cones evenly over the perimeter of the top layer of cake. Tear off pieces of cotton candy and cover the middle of the cake to represent unicorn’s mane. Scatter some extra sprinkles over the top of the cotton candy, and press some along the base of the cake to create some extra drama!