Pan di Spagna (Sponge Cake)

Method

1. Separate the eggs, placing the whites in the bowl of a stand mixer and the yolks in a separate small bowl.
2. Add 4 teaspoons of the total sugar to the egg whites and whip on high speed until stiff peaks form.
3. Remove about one quarter of the whipped whites and set aside in another bowl. (The reserved whites will be folded in later.)
4. To the remaining whites in the mixer bowl, add the egg yolks, the remaining sugar, vanilla extract (about 1½ tsp), rum (a splash), and grated lemon zest. Whip on medium-high speed for about 10 minutes, until the mixture becomes pale, thick, and triples in volume.
5. Fold the reserved whipped whites into the batter: start by adding one large spoonful of the reserved whites and fold gently with a spatula. Then alternate adding the remaining reserved whites and the sifted flour, folding from bottom to top. Reserve a small amount of the whites for the final addition.
6. After all the flour is incorporated, fold in the last portion of reserved whites to restore airiness.
7. Butter and flour a 32 cm (12.5 inch) round cake pan thoroughly.
8. Pour the batter into the pan and level the surface with a spatula.
9. Bake in a preheated oven at 160°C (320°F) for 30 minutes. Do not open the oven door during baking.
10. The cake is done when it begins to shrink away from the sides of the pan. Remove from the oven and let cool for a few minutes, then unmold.
11. Let the sponge cake cool completely on a wire rack. Serve as is or use as a base for other desserts.