Beat the egg with the milk and granulated sugar in a pot on the stove and stir until slightly thickened. Add the starch, pre-diluted in a little water and stir the cream until thickened. Remove from the heat and add 1 lump of butter.
Begin to stack the cake, by putting a little cream on the bottom of the pan or form you are using. Arrange a layer of biscuits dipped in milk, a cream layer, then sprinkle with walnuts, biscuits again and so on until out of products. Top the cake with cream.
To decorate, crush some biscuits with 1 tablespoon of cocoa and sprinkle this over the cake.