Soak the raisins in the rum for at least 1 hour. Heat the milk to boiling, add the sugar and stir until dissolved. Separately beat the eggs with the sugar and flour and start adding this mixture to the warm milk while stirring. Put the cream on the stove and stir until thickened.
Flavor with vanilla and add 2 tablespoons of rum-soaked raisins. Begin to stack the cake, alternating between a row of biscuits, a row of cream and so on, until out of ingredients. Between the layers, sprinkle on some drained raisins and walnuts.
Top with plenty of cream, decorate as desired. Let the cake stand at least 4 hours before serving.