Italian Sponge Cake (

Preheat the oven to 350u00b0F (180u00b0C). Lightly butter and flour one or two 9- or 10-inch (23- to 25-centimeter) springform cake pans.

Combine the whole eggs and sugar in a stand mixer and beat on high speed for 15 minutes, until the mixture quadruples in volume.

Add the egg yolks and beat on high speed for 5 minutes. The mixture should be thick enough to write with. Sift the flour and potato starch gradually into the egg-sugar mixture, incorporating a little at a time, mixing each inclusion of flour delicately with a spatula, scooping from the bottom up, so you do not deflate the eggs. Mix in the vanilla and lemon zest until just blended, then divide the batter between the prepared pans.

Bake for about 30 minutes, until light golden and a toothpick inserted in the center of the cake comes out clean. Do not overbake. It will dry and set as it cools.

Turn out onto a wire rack and let cool to room temperature.