Line a baking tray (approx. 35 x 40 cm) with baking paper. Separate the eggs. Beat the egg whites, 4 tbsp. cold water and 1 pinch of salt until stiff, while adding 100 g sugar and 1 packet of vanilla sugar.
Beat the egg yolks separately. Sift flour and baking powder on top and fold in. Spread on the baking tray. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 10 minutes.
Turn the sponge cake immediately onto a tea towel sprinkled with 2-3 tablespoons of sugar. Carefully remove the baking parchment. Roll up the sponge cake plate together with the tea towel from the long side. Let it cool down.
Soak gelatine in cold water. Wash and clean the strawberries. Puree 250 g strawberries. Mix with yoghurt, lemon juice and 50 g sugar. Squeeze the gelatine and dissolve at low heat. Stir in 3 tbsp. yoghurt cream and then stir into the rest of the cream.
Chill the yoghurt cream for about 15 minutes until it begins to gel.
Whip 300 g cream until stiff. Fold into the yoghurt cream. Unroll the sponge cake plate. Spread with strawberry cream, leaving about 2 cm free all around. Roll up the sponge cake, chill for about 3 hours.
Cut the rest of the strawberries into very thin slices. Whip 250 g cream and 1 packet of vanilla sugar until stiff. Spread the roll with it and cover with strawberry slices.