Prepare two rectangular baking trays approx. 23 x 20 cm or equivalent. Grease with margarine. Preheat the oven to 170 deg C.
Mix together ingredients for Pastry 1 (add a little more soured cream, if necessary) and put aside.
Melt margarine for Pastry 2 and stir in the flour to create a firm mixture.
Roll our Pastry 1 as thin as possible and spread Pastry 2 on top of Pastry 1.
Form a tight swiss roll and cut it in half. They will be the base and top of the cake.
Roll out each half to the size of the baking tray, covering the base only. The pastry will shrink slightly when in the oven, so if it is slightly larger (by a few cm), you can leave the excess pastry resting against the edge of the baking tray, without the need of trimming it. The rolled out pastry should be thin and will be delicate, so should be placed onto the baking tray using the rolling pin.
Bake each half separately for about 15-20 minutes or until golden brown.
To make the cream, split the milk into 600ml and 400ml portions. Stir in the sugar, flour, egg yolks and vanilla sugar into the 400ml part. Mix thoroughly using electric whisk for a smooth mixture. Transfer the mixture to a large saucepan.
Boil the other 600 ml portion of milk and once heated up, pour the hot milk into the cold mixture of milk, sugar, vanilla sugar and flour. Place on a hob (set to low heat) and continue heating up in order to make creme patisserie. Stir continuously ensuring the cream does not stick to the saucepan, otherwise the mixture will burn. The cream will be gradually getting thicker and will be ready once it reaches thick consistency and there are air bubbles visible. It is important you continue stirring at all times as the mixture is thickening.
Once ready, set the mixture aside to cool down completely. You can place the saucepan containing the cooked cream in a bowl with a little of cold water. Once the mixture is cold, stir in the butter using an electric whisk.
Use one of the baking trays for building the cake. Place the cream onto the base and cover with the top pastry. Decorate with icing sugar.
For best results place in fridge for 1-2 hours before serving.