Best-Ever Birthday Cake

Make the cake: Preheat oven to 350°F. Grease 3 (8-inch) round cake pans with cooking spray, and line bottoms with parchment paper. 

Beat egg whites in a heavy-duty stand mixer fitted with the whisk attachment on high speed until stiff peaks form, about 2 minutes. Transfer to a medium bowl; set aside. Change to the paddle attachment, and beat sugar and softened butter in same bowl on medium-high speed until light and fluffy, 3 to 5 minutes. Reduce speed to medium-low, and add egg yolks, 1 at a time, beating just until blended after each addition, stopping to scrape down sides of bowl as needed. Add vanilla, and beat on low speed just until incorporated.

Whisk together flour, baking powder, and salt in a medium bowl. Add flour mixture to butter mixture, alternately with buttermilk, beginning and ending with flour mixture, beating on medium speed just until combined after each addition. Stop mixer, and fold in beaten egg whites and sprinkles. Divide mixture evenly among prepared pans. 

Bake cake layers in preheated oven until a wooden pick inserted in center comes out clean, 20 to 25 minutes. Let cakes cool in pans on wire racks 10 minutes. Invert cakes onto racks. Remove and discard parchment paper, and let cool completely, about 30 minutes.

Make the frosting: Beat butter in stand mixer fitted with paddle attachment on medium-high speed until fluffy, about 3 minutes. Add sour cream, and beat until combined. Add cocoa, and beat on medium speed until combined, about 1 minute. Add vanilla, and beat on low speed just until incorporated. With mixer running on low speed, gradually add powdered sugar and salt, beating until just incorporated. Increase speed to medium-high, and beat until well blended, 2 to 3 minutes. 

Spread frosting between cake layers and over top and sides of cake. Garnish with additional sprinkles.