Separate eggs. Beat the egg whites with the whisk of the hand mixer until stiff, adding salt, 75 g sugar and vanilla sugar. Stir in egg yolks. Mix flour, starch and baking powder, sieve onto the egg mixture and fold in.
Spread the mixture evenly on a baking tray (34 x 38 cm) lined with baking paper. Bake in a preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: see manufacturer) for 8-10 minutes.
Take out the sponge cake and let it cool down.
Soak 7 leaves of gelatine in cold water. Wash apples, grate dry, quarter and remove core. Cut quarters into small cubes. Bring the cubes, lemon juice and apple juice to the boil in a small pot and simmer over a low heat for about 1 minute.
Squeeze out the gelatine and dissolve in it. Pour into a bowl, allow to cool and chill for about 45 minutes.
Soak 5 sheets of gelatine in cold water. Whip cream until stiff. Halve the vanilla pod lengthwise, scrape out the pulp. Stir mascarpone, 75 g sugar and vanilla pulp until smooth. Gradually fold in the cream.
Squeeze the gelatine, dissolve and stir in 4 tablespoons of the vanilla cream, stir into the remaining vanilla cream.
Turn over the sponge cake, carefully peel off the paper. Halve the sponge crosswise, place one half on a cake plate and enclose with a square cake frame (approx. 17 x 19 cm). Smooth the vanilla cream on the base, place a second base on top.
Chill the cake for about 30 minutes.
When the apple mixture begins to set, spread it on the cake. Chill for at least 1 1/2 hours. Carefully remove from the frame and cut into pieces.