In a metal bowl set over a saucepan of barely simmering water combine chocolate and water and stir until chocolate is melted and sauce is somewhat smooth, about 20 minutes. (Most of the water will have evaporated and mixture will have thickened slightly.) Remove bowl from heat and stir in cream and liqueur until combined well. Cool sauce completely and transfer to a jar with a tight-fitting lid. Keep the sauce, covered and chilled, up to 1 month. Serve sauce over ice cream.