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