For the dough, melt 250 g butter and let it get firm again. Cream with lemon zest, sugar and vanilla sugar with the whisks of the mixer. Stir in the eggs one after the other. Mix 125 g flour, starch and baking powder and stir into the egg cream.
Pour the dough into a greased, flour-blasted box form (25 cm long) and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for 55-60 minutes.
Cut the cake lengthwise with a knife after about 30 minutes of baking time. Bake to the end. Let the cake cool down a bit, turn out of the tin and let it cool down completely.
For the filling, whisk 350 g butter (at room temperature) and icing sugar with the whisk of the mixer for approx. 10 minutes until white-creamy. Stir in the Lemon Curd by the spoonful to create a smooth cream.
Stir the jelly until smooth. Cut the cake twice horizontally.
Spread the jelly on the lower base except for something to decorate. Spread 1⁄4 of the buttercream on top and smooth it down. Place the second base on top and also spread 1⁄4 buttercream on top.
Place the third bottom as a lid on top. Chill the cake for about 1 hour. Then spread the rest of the cream all around and sprinkle with brittle. Decorate with the remaining jelly and refrigerate again for about 2 hours.