Defrost berries in a sieve, collect juice. Preheat the oven for the cake bases (electric oven: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease the springform pan (26 cm Ø). Separate the eggs.
Cream 125 g butter, 125 g sugar and 1 vanilla sugar with the whisks of the mixer. Stir in the egg yolks one after the other. Mix flour, baking powder and lemon zest. Add milk and stir into the butter-sugar-cream.
Smooth half of the dough in the form. Sprinkle with approx. 1⁄3 berries.
Beat the egg whites until stiff, adding 200 g sugar. Spread half of the beaten egg white loosely on the dough in the mould (keep the rest of the beaten egg white cold). Bake in a hot oven for about 25 minutes. Remove, carefully remove from the tin and let it cool down.
Clean the springform pan and grease it again. Add the rest of the dough, smooth it down and sprinkle with approx. 1⁄3 berries. Fill the remaining beaten egg white into a piping bag with a star-shaped spout. Spray small tuffs onto the surface of the dough, close together.
Bake in a hot oven for about 25 minutes. Remove from the oven, remove from the mould and allow to cool.
For the cream, soak gelatine in cold water. Mix sour cream, remaining berries, 60 g sugar and 1 vanilla sugar. Warm up the blueberry juice in a small pot. Squeeze the gelatine slightly and dissolve it in the juice at low heat.
First stir in 2 tablespoons of blueberry cream, then stir the gelatine mixture into the rest of the blueberry cream. Whip cream until stiff, fold in and refrigerate for approx. 10 minutes.
Spread the blueberry cream on the first baked cake base. Place the cake base with meringue tuffs on top. Chill for about 1 hour.