Pre-heat the oven to 350F.
Lightly grease and flour 2 8-inch round cake pans, then line the base of each pan with parchment or baking paper.
Combine the eggs, sugar, vanilla, and lemon zest in a large mixing bowl. Use an electric mixer to beat until you have a thick creamy pale mix that has tripled in volume, about 10 minutes.
Sift the flour, use a wood or metal spoon to gently fold in the sifted flour to the creamy egg mix; it works best to add the flour in small batches.
Pour the batter, in equal amounts, into the cake pans and then gently tap/drop the pans on the counter to remove any large air bubbles.
Bake the cakes in the pre-heated oven for about 25 to 30 minutes or until the cakes are golden and springy – the edges of the cake will also start to separate from the pan.
Unmold the cakes and let them cool down completely. Right before assembling the layer cake, place each sponge cake on a flat surface and use a long serrated knife to cut through the middle (horizontally).