Mix all of the ingredients for the layer in a large bowl and stir. Pour the resulting biscuit dough onto a large sheet of aluminum foil and smooth it out into an even rectangular cake layer.
Put it in the fridge briefly. In the meantime, boil the milk. Add the chocolate pieces to it and stir the cream until thickened.
Then smear it over the layer and use the foil to wrap it up into a roll. Put the cake in the fridge to harden. Finally, cut into pieces and serve.