Preheat oven to 350°F. Line three 6” cake pans with a round of parchment paper on the bottom. Do not grease the pans. Set aside.
In a large mixing bowl, add the egg whites and cream of tartar. Use an electric mixer fitted with a whisk attachment to beat the egg whites until they become soft, foamy peaks.
Beat in one spoonful of granulated sugar at a time, mixing in between each addition, until all the sugar is added and the meringue becomes stiff peaks that hold their shape when you pull the whisk out of it. Set aside.
In a separate large bowl, add in the egg yolks, milk, oil, and vanilla extract. Use the same mixer to whisk together until combined.
Using a fine mesh sieve, sift in the cake flour and baking powder into the egg yolk mixture. Use a silicone spatula to fold together until smooth and no lumps remain.
Add half of the egg white meringue into the egg yolk mixture. Fold together until combined. Add the rest of the meringue and fold together until no streaks of batter are visible and the mixture is fluffy and airy.
Divide cake batter into your three prepared cake pans.
Place a second roasting pan filled with about 1” of hot, freshly boiled water on the bottom rack of the oven.
Bake cakes on the middle rack for 20–25 minutes or until the the tops are set and lightly golden brown. Let cool for at least 20-30 minutes before flipping the cakes out onto a cooling rack to cool completely before assembling the cake.