German Torte (with Lemon and Buttermilk)

Cake:

1. Preheat the oven to 200 degrees Celsius/ 400 degrees Fahrenheit. Line only the bottom of a springform (about 26 cm/10.2-inch diameter) with baking paper.
2. Egg whites: Separate the eggs. Whisk the egg whites until stiff. Slowly add half of the sugar and the vanilla sugar and whisk again until stiff and glossy. Set aside.
3. Egg yolks: In another bowl beat the egg yolks and the rest of the sugar until pale and creamy. Carefully fold the egg whites into the egg yolk mixture.
4. Mix the flour, starch, and baking powder and sift over the egg mixture. Fold in carefully.
5. Bake: Pour the batter into the prepared form and bake for about 25 minutes until golden brown. Remove from the oven, let stand for about 5 minutes, remove from the baking form and leave to cool completely on a wire rack.

Filling:

1. Soak the gelatine leaves for about 10 minutes (or according to the packet instructions) in cold water.
2. Combine: In the meantime, stir together buttermilk, sugar, lemon zest, and lemon juice.
3. Dissolve the gelatine according to the packet instructions. I squeeze it very gently and place it into a pan large enough to hold the buttermilk mixture as well. I heat the gelatine on the lowest heat while stirring all the time with a spoon. It will only take about 20-30 seconds for the gelatine to dissolve, do not let it boil.
4. Filling: Immediately start adding the buttermilk mixture to the gelatine, one spoonful after another, mixing continuously and only adding the next spoon to the mixture when the previous has been completely incorporated. Work quickly, and after incorporating about half of the mixture you can start adding a bit more than one spoonful at a time to the mix. Place this in the refrigerator for about 5-10 minutes or until it just starts to set. Don't let it set, though, or the mixture will turn clumpy when adding the whipped cream.
5. Beat the first 600 ml/ 2 ½ cups whipping cream until stiff and carefully fold into the buttermilk mixture.

Assemble and decorate:

1. Cut the base of the cake into two slices. Place one piece on a cake platter and place the clean ring of the springform (or a special ring for this kind of purpose) around the cake slice.
2. Assemble cake: Pour ½ of the buttermilk-whipping cream mixture on top, level with a spoon, and cover with the second cake slice. Pour the rest of the buttermilk mixture on top, level nicely.
3. Let set: Place the cake in the fridge until set, ideally overnight.
4. Release cake: When the cake is set, carefully go around between the cake and the springform ring with a knife. Remove the ring carefully.
5. Cream: Whisk the rest of the whipping cream slowly adding the stabilizer, if using any. Place a few tablespoons of the cream into a piping bag with a nice nozzle of choice. Spread the rest of the whipping cream on top and around the edge of the cake.
6. Decorate the cake with whipping cream from the piping bag, pistachios, and lemon slices.
7. Refrigerate until ready to serve.