Preheat the oven to 180 °C (fan oven 160 °C).

Separate the egg yolk from the egg white.

Whisk the egg yolks or apple purée with the water and Tagatesse until you have a light-coloured, fluffy mixture.

Tip: As Tagatesse can caramelise more quickly, beat the mixture at a low speed and avoid beating it for too long or too vigorously.

Sift the flour and potato starch and add them to the mixture, along with the salt. Mix gently.

Add the melted, but not hot (at most lukewarm) butter and stir it in.

If you’re using eggs, whisk the egg whites until they form soft peaks.

Gently fold the egg whites into the batter, along with the blueberries, to retain as much lightness as possible. If you’ve used apple sauce, skip this step.

Divide the batter between four greased cake tins.

Bake the cakes in the oven for about 25–30 minutes. Check they are cooked through using a skewer (the skewer should come out clean).