A classic sponge cake that despite its "spagna" name is actually Italian. The only leavener it uses is the good old-fashioned "get as much air into it as you can" method.
Let eggs come to room temperature before starting.
Butter and flour a 9" (23 cm) cake pan. Set aside.
Start heating your oven to 175 C / 350 F / Gas Mark 4.
Sift the flour into a bowl; set aside (if ever there's a recipe in which not to be cynical about sifting flour, this is the one. Just do it.)
Separate the eggs, yolks in one bowl; whites in another.
Add the sugar to the egg yolks, and whip with a hand mixer until thick. Stir in the lemon zest and vanilla.
Rinse the beaters off well, then use the hand mixer to whip the egg whites to a stiff peak.
Then, using a spatula or spoon, fold the beaten egg white into the yolk mixture.
Now add the flour a bit at a time to the egg mixture, and fold it carefully in.
Pour the mixture into the prepared cake pan, place in oven, and bake for about 20 minutes. When done, a toothpick will come out clean, and the top will be golden.
Turn upside down (sic) on a wire cake rack to cool.
When cool, ice and serve, or serve plain, or use as an ingredient in another recipe.