1: Preheat the oven to 360°F (180°C). Prepare the tin or mould by lining the base and sides with cooking parchment. You can scatter a few flakes almonds in the bottom if you like, but this is not essential.
4: Beat on high speed until the mixture is light like a mousse.
5: Sieve the flour and potato flour into the bowl.
6: Gently fold in the flour, just until it is incorporated into the frothy egg mixture. This is the tricky part of the recipe. This needs to be done rapidly but gently to keep as much air as possible in the mixture. It is best to lean the bowl over on its side and use a soft spatula to lift and turn the mixture.