Line the bottom of the springform pan (26 cm Ø) with baking paper. Separate the eggs. Beat egg white and 3 tbsp. water until stiff. Let the sweetness trickle in. Beat the egg yolk into the mixture. Fold in flour, starch and baking powder.
Paint into the mould. Bake in a hot oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2) for approx. 20 minutes. Cool down.
Soak 6 leaves of gelatine. Season yoghurt, peel and juice with sweetener. Dissolve gelatine. First stir in 4 tablespoons of yoghurt, then stir into the rest of the yoghurt. Whip cream until stiff, fold in.
Cut the sponge in half. Place the edge of the mould around 1 base. Fill in half the yoghurt cream. Place 2nd base on top and spread the rest of the cream on top. Chill for about 1 hour. Soak 3 sheets of gelatine, dissolve. First stir in 4 tbsp. groats, then stir into the rest of the groats.
Spread on the cake and chill for at least 4 hours.