Melt the butter and mix it with the crumbled biscuits. Stir well. Cover the bottom of a form with the resulting mixture. Put it in the freezer for 10-15 min. for the layer to harden.
Beat the cream with the ground walnuts. Pour the resulting mixture over the bananas. Leave the cake overnight in the fridge. Before serving, you can decorate the sides with banana slices and add a chocolate glaze on top.