Arrange the biscotti tightly in a cake dish (10″ (25 cm)).
Beat the soft butter with the sugar, eggs, and the cocoa, pre-beaten in 1 cup of hot water and the rum. From this mixture, set aside 1 cup to smear on the outside of the cake.
Lightly squish the meringues by hand and along with the coarsely crushed roasted walnuts, add them to the main cream mixture. Gently mix.
Arrange the cake in the form, coat it with the cream that was set aside and place it in the refrigerator.