Using a hand mixer, beat the egg whites until stiff peaks form. Set aside.

In a separate bowl, beat the egg yolks and sugar until pale and fluffy.

Bring a pot of water to a boil. Once boiling, remove it from the heat and place the bowl with the whipped yolks on top. It should sit over the steam but not touch the water, like a double boiler.

Gradually add the flour, potato flour, and lemon zest, whisking continuously until smooth and fully combined.

Gently fold the beaten egg whites into the yolk mixture using a spatula, keeping as much air as possible in the batter.

Grease a springform pan and dust it lightly with flour. Pour the batter into the prepared pan and smooth the top.

Bake in a preheated oven at 190°C – 375°F for 30 to 35 minutes, or until a toothpick inserted in the centre comes out clean.

Allow the cake to cool completely before unmoulding. Once cold, slice it horizontally and fill with jams or creams, or use it as a base for trifles and other desserts.