Preheat oven to 350 degrees
Grease a 9x13 baking pan with butter
Combine wet ingredients in a large bowl, combine egg yolks, milk, vanilla and melted butter.
4 eggs, 1 cup milk, 1 tsp pure vanilla extract, 3/4 cup salted butter
In a medium bowl, combine flour sugar, baking powder, salt and add to the wet ingredients. Stir together but do not overmix.
2 cups all-purpose flour, 1 1/2 cups granulated white sugar, 1 tbsp baking powder, 1 tsp salt
In a separate bowl, beat the egg whites until stiff and a peak forms. Use a spatula to fold the egg whites into the batter.
4 eggs
Pour batter into pan and bake for 30 minutes
Cut into squares to serve