Cut the sponge cake into strips and use it to line a spherical mold. Finely mince the chocolate and melt half of it in a double boiler over a medium heat. Pour the whipped cream in a bowl. Add 3 oz. confectioner's sugar, vanilla, curaçao and minced nuts. Lightly moisten the sponge cake with liqueur in the mold and spoon in half of the whipped cream.
Mix the remaining whipped cream with the chocolate. Scoop this into the mold, filling it to the top. Seal with more sponge cake strips and moisten with the liqueur. Put the zuccotto in the freezer for about 4 hours. Make a funnel of wax paper and fill it with melted chocolate.
Make small disks of chocolate on another piece of wax paper and let harden. Remove the zuccotto from the freezer one hour before serving. Unmold the zuccotto onto a serving dish, sprinkle with powdered sugar and garnish with chocolate disks and cocoa powder if desired. Slice and serve.
