Preheat the oven to 350° F. Grease a 9x13 pan with butter and set it aside.
In a bowl, whisk to combine the flour, baking powder, and salt. Set it aside.
1 cup all-purpose flour, 1½ tsp baking powder, ½ tsp salt
In a separate bowl, place the egg yolks and add 3/4 cup of the sugar. Whisk until the two are combined and yolks turn a lighter shade of yellow. Set the yolks aside.
5 large eggs, 1 cup granulated sugar
In another bowl, place the egg whites and the remaining sugar. Using a hand or stand mixer, beat the whites on high until stiff peaks form. Set them aside.
5 large eggs, 1 cup granulated sugar
Form a well in the center of the dry ingredients and add the yolk mixture. Pour in the milk and stir until a batter forms.
⅓ cup whole milk
Add a third of the whipped egg whites to the batter and carefully fold them in until no streaks remain. Repeat with the remaining 2/3 of the egg whites.
Pour the finished batter into the prepared pan and smooth the top using an offset spatula. Place the cake in the oven and bake until golden, about 25-30 minutes.
Take the cake out of the oven and let cool completely.