For the cream, pour 3 1/3 cups (800 ml) of the milk in a casserole, put it on the stove and heat it up. Add the vanilla, butter and sugar, leave to boil.
Heat the rest of the milk just a bit and while stirring nonstop add the eggs and flour, taking care to avoid lumps from forming.
Once the milk in the casserole comes to a boil, pour the egg mixture in a very thin trickle to it and beat vigorously nonstop to obtain a smooth, fluffy cream.
Start assembling the cake: arrange a layer of plain biscuits in a rectangular tray, pour on a few ladles of the hot cream, even it out, arrange a layer of cocoa biscuits, cream again and so on to the end, finishing off with cream.
Notes: You don't need to syrup the biscuits if you pour the still hot cream over them. Otherwise, you can syrup them with compote juice.
You can use a round form if you don't have a suitable rectangular one. Fill the gaps with crumbled biscuits like I did. :)