How to make
Combine the egg, sugar and vanilla sugar in a bowl, beat with a mixer to foam. Add them to the cold milk, add the starch, stir until smooth.
Put the mixture on low heat on the stove, stir nonstop with a spatula and boil to a thickened cream. Cover the cream with plastic wrap and leave it to cool completely.
Dissolve the instant coffee in 1 cup warm water and leave it to cool. Separately, beat the sour cream, add the cold cream to it in lots to obtain a smooth mixture.
Try the cream and if it's not sweet enough, you can add powdered sugar as desired and stir again (I didn't add any to mine).
In the container you've prepared for the cake, arrange biscuits as desired. In my case I had 9 biscuits per layer (3 x 3). Arrange the cake in this order: biscuit layer, pour on 1 tsp coffee, smear with cream.
Sprinkle with grated chocolate on top and leave in the fridge to set for a few hours before consumption.
