In a stand mixer, beat the 5 eggs (no need to separate them) and sugar on low speed and then on medium-high speed for about 10 minutes or until it has tripled in volume.
Using a spatula, fold in the flour in thirds. Be very gentle with the batter so you don't over mix it and lose the fluffiness.
Pour the cake batter to an oiled baking pan. A springform baking pan that is at least 9 inches works amazing.
Bake at 350°F for 30 minutes and check it using a wooden toothpick. If the cake is ready, the toothpick will come out clean.
Notes
Notes:
This recipe doesn't need baking soda or baking powder. The leavening agent is the air that is beat into the batter.
The sponge cake may have a slight eggy taste, but if you beat long enough (at least 10 minutes) this shouldn't happen.
The sponge is best used as a base for making other cakes where you would add frosting, fillings, etc. If you prefer, you can add some vanilla extract in step 2.