Preheat oven to 350°F (177°C). Grease two 9-inch round cake pans, line with parchment paper rounds, then grease the parchment paper.

Whisk the cake flour, baking powder, baking soda, and salt together in a large bowl. Set aside.

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter and sugar together on high speed for 3 full minutes.

Turn the mixer down to medium-high speed and beat in the 4 egg yolks one at a time, letting each egg yolk fully mix into the batter before adding the next. Beat in the vanilla extract until combined.

Add about ⅓ of the dry ingredients and ½ of the buttermilk and beat on low speed until combined. Add ⅓ more of the dry ingredients and the rest of the buttermilk and beat on low speed until combined. Add the rest of the dry ingredients and beat on low speed until combined.

Using a handheld or stand mixer fitted with a whisk attachment, beat the egg whites and cream of tartar together on high speed until fluffy peaks form, about 3 minutes.

Slowly and gently fold the egg whites into the thick cake batter.

Pour/spoon batter evenly into cake pans.

Bake for around 27-31 minutes or until the cakes are baked through.

Allow cakes to cool completely in the pans set on a wire rack.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes.

Add confectioners’ sugar, cocoa powder, 3 Tablespoons heavy cream, salt, and vanilla extract. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute.

If cooled cakes are domed on top, use a large serrated knife to slice a thin layer off the tops to create a flat surface.

Place 1 cake layer on your cake turntable, cake stand, or serving plate. Evenly cover the top with frosting. Top with 2nd layer.

Spread remaining frosting all over the top and sides. Garnish with sprinkles, if desired.

For extra neat slices, refrigerate cake for 30-60 minutes before slicing.