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Put a dollop of frosting on a 7-inch round cake board (or cake plate) or 8-inch scalloped cake board.
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Put your first layer top-up on the cake board or plate, and spread about 1 cup of frosting evenly across layer. Put the second cake layer on top and repeat with another layer of frosting. Put the final cake layer top-down. Cover the cake with plastic wrap and wiggle the layers into place. Refrigerate the cake for about 30 minutes.
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Using a turntable, if possible, frost entire outside of cake with a thin layer of frosting to seal in the crumbs. Chill until the frosting begins to firm-up, about 15 minutes. Repeat with another thin layer of frosting, this time working to achieve a smooth finish. Chill for another 15 minutes.
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Apply a third coat of frosting to the cake. Holding a tall pastry comb in your dominant hand, press it gently against the side of the cake and keep it steady. Use the other hand to slowly rotate the turntable until you have gone all the way around the cake. Gently press some chocolate sprinkles around the bottom edge of the cake.
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Fit a medium pastry bag with a medium-large closed star tip, such as 1M, and fill about 2/3 full with frosting. Pipe a border around the top of the cake. Chill the cake until the frosting border firms up, at least 30 minutes.
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Cover the top of the cake (but not the piped border) with chocolate sprinkles. Once frosting softens again, the sprinkles will adhere to top. The cake will keep refrigerated for up to 3 days.