28 cm sponge cake base to fill, for 15 people

I crack the eggs, which should be at room temperature, into a saucepan.

I whisk, put over a medium heat and continue stirring, measuring the temperature until it reaches 40/45°C. This will take 2/3 minutes.

I put the mixture in the stand mixer with the salt and start whisking at medium speed, gradually increasing, for about 10/12 minutes to incorporate air. As there is no baking powder, the air provides the structure.

The mixture should increase 5 times in volume and become pale and foamy. I test with the whisk: it should leave a ribbon on the surface for a few seconds.

Meanwhile I have sifted the flour and potato starch together 2 times.

I lined the base of the tin with baking parchment and buttered and floured the sides.

I add the flour in 3 additions, folding from the bottom upwards with decisive movements until completely incorporated. I added grated lemon zest.

I pour into the tin and level with a spatula without tapping the tin.

I bake in an oven preheated to 180°C for 30/35 minutes, using a fan oven. The original describes placing a small ball of foil in the door to leave a slight gap for steam to escape, to stop the cake rising and then collapsing.

If it browns too much, lower the temperature a few degrees, without opening the door. At the end I insert a skewer into the centre; it should come out dry. I remove the tin, sprinkle with a little granulated sugar and wait 5 minutes.

I turn the cake onto a rack, remove the parchment and leave to cool upside down. If not completely level, trim with a knife.

When completely cold, as I was not using it until the next day, I moved it onto a cake board and wrapped it entirely in cling film. The original says it can be kept in the fridge for 2/3 days, or outside the fridge for a day if the room is not hot.