• Grease and flour three 8" round cake pans. Line with parchment paper and grease and flour again. Preheat oven to 350 degrees F.
• Pulse the 1/2 cup of the sprinkles once or twice in a food processor.
• In a bowl, whisk the egg whites, milk and vanilla.
• In the bowl of your stand mixer, fitted with the paddle attachment, combine the flour, sugar, baking powder, and salt. 
• Add the 1/2 pound butter, with the mixer running on low, until all butter is combined and the mixture resembles crumbs.
• Slowly pour in all but 1/2 cup of the egg mixture, until just combined. Increase the speed to medium high and beat 1 minute, until light and fluffy. 
• Add in the remaining egg mixture. Scrape down the sides as needed and combine again on medium high heat for 15 seconds longer. 
• Fold in the sprinkles.
• Divide the batter evenly in the pans and smooth tops with a spatula. 
• Bake in the preheated oven for 21 to 25 minutes, until a toothpick inserted in the center comes out clean.
• Let cool on wire rack for 10 minutes before turning out cakes and removing parchment. Cool Completely.

Meanwhile, make the icing.

• In a clean bowl for your stand mixer, whisk the butter, heavy cream, salt, and vanilla with the whisk attachment. 
• Carefully add in the confectioners' sugar until all is incorporated. 
• Increase spread to medium-high and whip until fluffy, about 5 minutes.