Put the eggs and sugar in the bowl of a stand mixer and whisk on high speed for about 10 minutes. The mixture should triple in volume and be very foamy.
Sift the flour and cocoa over the mixture.
Fold in gently with a spatula, lifting the batter as you go. Do not overmix or the sponge may fail to rise in the oven.
Pour the batter into a buttered and lined tin
25 minutes at 180°C
Check the sponge is done by inserting a knife. If the blade comes out dry, it is baked.
Leave to cool, then cut three discs of equal thickness.