Mix the flour and baking powder and place in a bowl. Add 200 g butter in pieces, eggs, 200 g sugar and vanillin sugar. Mix with the whisks of the hand mixer for about 2 minutes to a smooth dough. Grease the fat pan of the oven (32 x 39 cm). Put the dough on top and smooth it down. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 20 minutes.
Let them cool down. For the cream, stir the pudding powder with 1 cup of water (200 ml) and egg yolk until smooth. Bring 1 cup of water (200 ml) to the boil with 1 cup of sugar (200 g), remove from heat. Stir in the pudding powder. Bring to the boil while stirring once. Remove from heat and allow to cool slightly. Stir 125 g butter in pieces and juice of 2 lemons into the cooled pudding until the butter has melted. Pour immediately onto the cake and smooth it down. Put in a cool place for about 1 hour. Whip the cream until stiff, pour onto the cold cream and smooth it down. Place butter cookies on the cream. Stir icing sugar and 6-7 tbsp lemon juice until smooth.
Pour immediately onto the cake and smooth it down. Put in a cool place for about 1 hour. Whip the cream until stiff, pour onto the cold cream and smooth it down. Place butter cookies on the cream. Stir icing sugar and 6-7 tbsp lemon juice until smooth. Spread over the butter biscuits and spread. Put in a cool place overnight. Serve decorated with lemon balm and jelly lemons