Prep: Preheat the oven to 350°F. Line three 6" round pans with parchment paper rounds on the bottom of the pans only. Set aside.
Separate eggs: Separate the egg whites and egg yolks into two large mixing bowls, being careful not to get any of the yolks in the egg whites. Set the bowl of egg yolks aside.
Beat egg whites: To the egg whites, add cream of tartar. Use an electric hand mixer on low speed to beat the egg whites until they become frothy like cappuccino foam.
Whip to stiff peaks: Sprinkle in the sugar, one spoonful at a time, mixing well in between each addition. Continue beating the mixture until it becomes stiff peaks (when you pull the mixer out of the bowl, the meringue should stand up in pointy peaks with a slight curl at the tips). Set aside.
Egg yolk mixture: To the egg yolks, add the milk, oil, and vanilla extract. Use the same hand mixer to mix until combined. Use a fine mesh sieve to sift in the cake flour and baking powder. Mix again until combined and smooth.
Combine egg whites and egg yolk mixtures: Add ⅓ of the egg white meringue into the egg yolk mixture. Use a spatula to fold them together until mostly combined. Add the rest of the meringue and fold until no streaks remain. Be sure to scrape the bottom and sides of the bowl where thicker batter settles.
Fill pans: Divide the batter into your lined cake pans (about 185-200g of batter per pan). Lift and drop each pan on the counter to get rid any large air bubbles.
Bake: Bake cakes for about 25-27 minutes or until the tops are lightly golden brown. Turn the pans upside down on a wire rack and let them cool completely.
Release cakes: Once cooled, run an offset spatula around the edges of the cake and turn the cakes out. Peel the parchment paper off the cake layers.