First, make the cream. Pour about 3 1/3 cups (800 ml) of the milk in a casserole. Stir the egg, sugar, vanilla sugar, pudding and flour well. Pour the rest of the milk (cold) into this mixture.
Once the mixture becomes homogenized, pour it in a thin trickle into the milk (hot but not boiling). Stir with a whisk until the cream bubbles.
After it cools slightly, it's ready to pour over the biscuits. Pour a ladle of the cream into a suitable form, even it out and arrange a layer of biscuits.
Next put cream, biscuits again, cream and finish off with biscuits, pressing them a bit so cream pops out in the middle if they have holes.
Once cooled, the cake is ready. You can sprinkle powdered sugar on top.