Instructions:
Preheat Oven: Set your oven to 325°F (160°C). Line an 8-by-8-inch baking pan with parchment paper.
Prepare Egg Yolk Mixture: In a large bowl, beat the egg yolks and powdered sugar until pale and creamy, about 2-3 minutes.
Incorporate Butter and Vanilla: Gradually mix in the melted butter and vanilla extract into the egg yolk mixture, beating until well combined.
Add Flour: Sift the flour over the egg mixture and beat until fully combined.
Add Milk: Gently stir in 1 tablespoon of warm milk until the batter is smooth and well-mixed. The batter will be quite thin.
Beat Egg Whites: In a separate bowl, beat the egg whites until stiff peaks form.
Combine Mixtures: Carefully fold the beaten egg whites into the batter in three additions. The batter may look a bit lumpy, which is normal.
Prepare Pan: Spray the prepared baking pan with cooking spray and smooth the batter evenly in the pan with a spatula.
Bake: Bake in the preheated oven for about 50-60 minutes, or until the cake is golden brown and a toothpick inserted into the center comes out clean.
Cool and Dust: Allow the cake to cool completely in the pan before dusting with powdered sugar.