Take the eggs out of the refrigerator ahead of time as the eggs need to be at room temperature or soak them for a few minutes in a bowl of warm water.
Preheat the oven to 170 degrees C (338 degrees F).
Butter and flour (or spray with baking spray) a 20 cm (8 inches) pan.
Put the eggs, sugar, salt, and lemon zest in the bowl of your stand mixer.
Beat the eggs until very fluffy and pale yellow (about 15/20 minutes on medium/high speed). To test that it has been beaten enough, let a small amount of batter fall into the bowl. If it remains "sitting" on top, it means that it's ready.⏱ 20 min
Sift the flour on top of the egg mixture, a little at a time, and fold it gently with a wooden spoon.
Pour the batter into the prepared pan. Don't smooth the top or bang the pan on the counter, leave it as it is!
Bake for 40 minutes or until a toothpick inserted into the center comes out clean. (Remember: do not open the oven for the first 30 minutes!)⏱ 40 min
Turn off the oven but leave the cake inside (keep the door slightly open using a wooden spoon) for 5/10 minutes so it can cool down slowly.⏱ 8 min
Remove the sponge cake from the oven, let it cool for 10 more minutes, then loosen around the edges with a knife, and flip it on a wire rack upside down (without the pan) to cool completely.⏱ 10 min
Enjoy it!