Place the cake on a silver or fireproof dish and soak with liqueur. Whip the egg yolk and sugar until thick and fold in the stiffly-beaten egg whites. Add vanilla essence and place the soufflé mixture in a piping bag with a large rose pipe. Place the ice cream on top of the cake. Completely cover with piped soufflé mixture. Dredge with icing sugar and place in a very hot oven for 3 min. Decorate and serve immediately.