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Preheat oven to 350°F/171°C. Grease and line two 7-inch round pans. Set aside.
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In a bowl, combine cake mix, oil, water, eggs and sprinkles. Beat until well combined.
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Evenly divide the cake batter among the prepared pans.
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Bake for 20-25 minutes or until a toothpick inserted at the center comes out clean. Remove from the oven. Cool first before frosting.
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For the frosting, in a mixer bowl over a double boiler. Add in egg whites and sugar. Stir just until sugar melts.
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Place mixer bowl on the stand mixer with a whisk attachment, beat on medium high speed until it forms stiff peaks.
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While the mixer is running, add in the cubes of butter one at a time. Continue whipping the buttercream until it forms a smooth consistency.
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Level off the top of the cakes. Place one layer on a serving dish, spread some frosting as filling, top with the other cake layer.
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Frost the cakes thinly, this will be the crumb coating. Refrigerate for a few minutes for the buttercream to set.
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Remove the cake from the fridge, coat with the final layer of frosting.
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Coat the cake with candy sprinkles, lightly press to secure sprinkles. Serve.