Preheat oven to 350°F (175°C). Grease and flour two 8-inch round cake pans.
Mix and bake cake according to box directions. Bake 25–30 minutes or until a toothpick comes out clean. Cool 10 minutes in pans, then cool completely on a rack.
Tint frosting with blue gel until you get a pale icy blue. Place one cake layer on a plate and spread 3/4 cup frosting.
Top with second layer and crumb-coat with 1/2 cup frosting. Chill 15 minutes.
Finish frosting with remaining buttercream, smooth with an offset spatula.
Press sanding sugar on the sides and top for sparkle. Roll fondant thin and cut snowflakes, or use sprinkles and pearls for accents. Arrange snowflakes on top and edges.