Melt the chocolate in a heatproof bowl set over a saucepan of gently simmering water. Meanwhile, line a 1 kg (2 lb) loaf tin with clingfilm so that the base and sides are covered.
Whip the cream until softly peaking. Fold in the icing sugar then the melted chocolate. Spoon a thin layer into the base of the lined tin.
Mix the brandy or coffee liqueur and cooled coffee in a shallow dish. Dip the sponge finger biscuits, one at a time, into the mixture to moisten then arrange in a single layer on top of the chocolate cream in the tin. Cover with half the remaining cream, then a second layer of dipped biscuits. Repeat the layers until all the cream and biscuits have been used.
Chill for 4 hours or overnight if preferred. To serve, loosen the edges and invert on to a serving plate. Peel off the clingfilm. Whip the double cream and spoon over the top, then dust with cocoa powder. Cut into thick slices to serve. Refrigerate for up to 2 days.