Line a 1.2litre/2pint round-bottomed bowl or pudding basin with damp muslin or cheesecloth, leaving it hanging over the rim.
Chill for 30 minutes. Reserve the remaining sponge fingers and liqueur.
Whip the cream with the icing sugar until very thick and standing in stiff peaks. Transfer three-quarters to a separate bowl. Stir the nuts and chocolate chips into the remaining one-quarter, then spread this over the sponge fingers to make an inner lining. Return to the refrigerator. Put the chocolate pieces and remaining 3 Tbsp liqueur in a heatproof bowl. Stand the bowl over a pan of gently simmering water and heat gently, stirring occasionally, until the chocolate has melted and is smooth. Remove from the heat and leave to cool slightly, then <
Press the sponge fingers down firmly, then cover with the muslin.
Carefully remove the muslin. Decorate with alternate sections of sifted cocoa powder and icing sugar. Fold a circle of greaseproof paper into 8 sections, then cut out each alternate one. Place over the Zuccotto and sift over the cocoa powder, then move the paper around so that the cocoa powder is covered and sprinkle with the icing sugar. If you are in a rush the Zuccotto can, of course, be simply dusted with either icing sugar or cocoa instead of both but the decoration is worth the effort for a special occasion.