Genoise Sponge Cake

According to Wikipedia a Genoise Sponge Cake is an Italian sponge cake that uses air beaten into the whole eggs to give volume to the cake and is a more elastic dry cake perfect for serving with buttercreme.

IngridCreates at http://ingridcreates.com/best-genoise-sponge-recipe/ has this great recipe, and we halved the quantities because we didn't need all the layers like she did.

Ingredients
4 eggs
125g caster sugar
125g sifted plain flour
50g melted unsalted butter (cooled)
1 teaspoon vanilla
(add 2 tabelspoons coco powder if you want Chocolate Sponge)

Directions:
Grease and flour pan, preheat oven to 180C.
In an electric mixer, beat the eggs and sugar and vanilla until it is very thick and frothy, which will take around 6 minutes. The volume of batter will double at this stage.
Sift again the flour onto the egg mixture and fold though
Trickle a steady stream of cooled melted butter down the side of the mixing bowl, not in the centre, and fold through with a light hand, until well mixed.
Bake in oven for 20 mins or until the top feels springy to touch or a skewer comes out clean. Don’t open the oven door before 15 minutes of baking time has lapsed or your cake will sink!
When baked, remove sponge cakes from oven and allow to cool in the tin for a few minutes, remove from the pan onto a cooling rack and leave until completely cold. 
Ingrid suggests to cover the cakes in a light muslin towel so they don’t dry out.