01 -
Heat oven to 350°F/180°C. Butter and line three 6-inch cake pans with parchment paper.
02 -
In a large jug combine and whisk together egg whites, whole egg, milk, and vanilla extract. Set aside.
03 -
In electric mixer bowl with paddle attachment, combine dry ingredients at low speed for 1 minute. Add cold butter and mix until mixture is a fine crumbly texture.
04 -
Add liquid mixture to dry ingredients in 2 batches, scraping sides and bottom before and after each batch. Mix until light and fluffy.
05 -
Divide batter evenly into prepared pans. Bake 25-35 minutes until tester comes out with few crumbs.
06 -
Let cakes cool on racks for 20 minutes before removing from tins.
07 -
Level cake layers. Layer cake with buttercream between each layer. Apply crumb coat, chill 30 minutes, then final coat. Decorate with sprinkles.