Oil and line a 20cm square tin or a round springform tin. Put the cream, chocolate and butter in a bowl set over a pan of simmering water. Avoid the bowl touching the water. When almost melted, after around seven minutes, remove from the heat, leave a few minutes and stir until smooth. Put the amaretti, nuts and prunes in a bowl and pour over the chocolate mixture. Turn gently to mix, then put into the prepared tin and cover with cling film. Chill in the fridge for around three hours, then dust thickly with cocoa powder, before cutting into slices