Place chocolate and hot water in the top of a double boiler, set over simmering waster, and heat and stir until chocolate is melted. Lightly beat yolks, add a little hot mixture, then stir back into pan and heat and stir over simmering water 1-2 minutes until no raw taste of eggs remains. Off heat, mix in vanilla. Beat cream until very thick and glossy (it should almost form soft peaks)and fold into chocolate mixture. Beat egg whites until soft peaks form and fold in, a little at a time, until no flecks of white show. Spoon into a serving dish, cover and chill at least 12 hours. Scatter with chocolate curls, if you like, just before serving.