Instructions

Weigh the eggs. They should weigh about 60 grams each.
Weigh the same amount of all-purpose flour: ca. 1 kg.
Weigh the same amount of sugar: ca. 1 kg.
Weigh the same amount of butter: ca. 1 kg.
You can use unsalted and salted butter (50-50), or unsalted butter alone with a good amount of salt (up to 1 teaspoon).
Pour the sugar in a large bowl. Add the soft butter and cream until white and fluffy with an electric mixer.
Separate the egg yolks from the whites. Set the whites aside.
Add the egg yolks to the sugar-butter mix. Stir well (with the wooden spoon). The more energetically you beat, the fluffier the cake.
Slowly add the sifted flour and incorporate it gradually as it falls on the batter.
Add some salt (up to a four teaspoons). This isn't needed if you are using salted butter.
Add a pinch of salt to the egg whites and beat until stiff. If the egg whites have been beaten enough you should be able to flip the bowl upside down and they won't fall down.
Incorporate the egg whites to the batter, one big spoonful at a time, very delicately, making under-and-over motions until evenly blended. The foam (air bubble) shouldn't "break". This is also a very important step in getting a fluffy cake.
Pour the batter in a metallic cake mold lined with baking paper.
Bake at 180C for about 45 minutes or until baked (if you stick a piece of uncooked spaghetti in the center of the cake it should come out dry).