Preheat the oven to 350 °F and butter a 9x13 casserole pan. Get out three mixing bowls.
In a large bowl, sift together flour, baking powder, and salt.
1 Cup Unbleached All-Purpose Flour, 1 ½ tsp Baking Powder, ¼ tsp Sea Salt
With the other two bowls, separate egg whites and egg yolks.
5 Large Eggs
Beat egg yolks with ¾ cup sugar with a stand mixer on high speed. Beat the egg yolks are a pale yellow (about 2 minutes).
1 Cup Granulated Sugar
Stir in milk and vanilla to your beaten egg yolks.
⅓ Cup Whole Milk, 1 tsp Vanilla Extract
Pour the egg yolk mixture over the sifted dry ingredients and fold in the egg yolks gently. Do not overmix.
Clean your stand mixer bowl and prepare to whip your egg whites.
Beat egg whites on high speed until soft peaks form (1 minute).
Pour the remaining ¼ sugar into the softly beaten egg whites. Beat on high speed until the egg whites are stiff but not dry (1 minute).
Gently fold in the egg white mixture with a spatula into the cake batter. Fold egg whites until just combined.
Pour batter into your prepared casserole dish and spread to even out the surface of the cake. Bake at 350 °F for 30-35 minutes or until a toothpick comes out clean.