Preheat the oven to 350°F. Spray two 8-inch round cake pans with nonstick cooking spray, or prepare with GOOP, and line bottoms of the pans with parchment paper. Set aside.
In a medium bowl, whisk together both kinds of flour, baking powder, and salt. Set aside.
In a microwave-safe bowl, add the milk and butter. Heat until butter has melted (20-30 seconds). Cover to keep warm.
Separate three eggs. In the bowl of a stand mixer, add the three egg whites only. To the egg yolks, add the other two whole eggs and set aside.
At low speed, beat the egg whites until foamy. Increase the speed to medium and gradually add half of the sugar. Beat until soft peaks form. (Soft peaks will briefly stand on their own before falling back in the mixture.) Transfer the beaten egg whites to a large bowl.
In the bowl of the stand mixer, add the egg yolks and whole egg mixture. Beat this mixture at medium-high speed with the remaining sugar until the eggs are thick, about 5 minutes. Add these eggs to the beaten egg white mixture.
Sprinkle the flour mixture over the egg mixture, gently folding about 12 times with a rubber spatula, or until mostly combined.
In one side of the batter, make a well. Pour the milk and melted butter mixture into the batter and continue to gently fold until there is no trace of flour remaining.
Pour the batter into the prepared cake pans. Bake for 16-18 minutes, or until the cakes spring back when touched. (Check a few minutes early for doneness.)
When done, remove the cakes from the oven and immediately loosen the cakes from the pans with an offset spatula. Invert each cake onto a large plate or serving platter. Remove parchment paper and flip each cake over again onto a wire rack. Let the cakes cool to room temperature before continuing with the recipe.