Dissolve the coffee granules in a pan in the hot water. Add the sugar and heat gently until the sugar dissolves. Bring to the boil and boil rapidly for 1 minute. Remove from the heat and leave to cool. Stir in the whiskey.
Melt the chocolate with half the cream in a bowl over a pan of simmering water (see
Spread a thin layer of the chocolate mixture on to a flat plate, to make a 23 x 10 cm (9 x 4 inch) rectangle. Dip one-third of the biscuits or sponge fingers in the coffee syrup so they are softened but not soggy.
Arrange the dipped biscuits or fingers side by side on the chocolate, then spread over more chocolate mixture. Dip half the remaining biscuits or fingers in the syrup and arrange them as before over the chocolate. Spread more chocolate mixture on top, then cover with the remaining syrup-dipped biscuits or fingers.
Spread the remaining chocolate mixture over the top and sides of the cake to cover it completely, smoothing with the knife. Chill for 3–4 hours until set. Decorate with chocolate curls and dust with cocoa powder.