According to legend, zuccotto is the first semifreddo (an Italian frozen dessert) in the history of cooking and was originally made in an infantryman's studded helmet. (In the Tuscan dialect, zucca means 'head.') It was originally prepared with ricotta, glacé (candied) fruit, almonds, and dark (semisweet) chocolate.
Line a freezerproof semi-circular mould with clingfilm (plastic wrap). Cut the cake horizontally into 2 rounds of equal thickness. Divide 1 of the rounds into 8 wedges and use to line the bottom and sides of the mold. Mix the liqueur with a little water and sprinkle it over the cake slices.
Whisk the cream in a bowl, gradually adding the sugar, until stiff peaks form. Spoon one-third of the cream into another bowl and fold in half the unsweetened cocoa powder. Crumble the amaretti into the remaining cream and spoon into the mould, gently spreading it over the side and leaving a small hollow in the center. Spoon the cocoa cream into the hollow and smooth the surface. Put the remaining cake round on top, cover and freeze for at least 6 hours.
Turn out the zuccotto onto a serving dish and remove and discard the clingfilm. Put the remaining unsweetened cocoa powder into a small sieve (sifter), sprinkle it over the dessert and serve immediately.