Take the eggs out of the fridge at least 2 hours ahead or place them into a bowl with warm water so they reach room temperature.
Preheat the oven to 340F.
Grease the pan with some soft butter and then flour it.
Place the eggs, sugar, lemon zest, salt and vanilla extract into the bowl of the mixer.
Beat on medium-high speed for about 15-20 minutes until they become pale and silky.
Sift the flour in a separate bowl.
Add 1-2 tablespoons of flour at a time to the egg mixture. Incorporate with a spatula or wooden spoon.
Transfer the batter to the pan.
Bake for 40 minutes or until a toothpick comes out clean. Do NOT open the oven until the baking time is done or your cake may collapse.
Turn off the heat and slightly open the oven door. Leave the cake like this for 10 minutes.
Take the pan off the oven and set aside for 10 minutes more.
Use a knife to unstick the edges from the pan and move the sponge cake to a wire rack to cool completely before cutting it.