1. Break the chocolate into small pieces and put it in a heatproof bowl with 30 ml (2 tbsp) cold water.
2. Stand the bowl over a saucepan of simmering water and stir until the chocolate melts.
3. When the chocolate has melted, beat it until smooth, gradually adding 200 ml (7 fl oz) water.
4. In a cup, mix the custard powder or cornflour with 30 ml (2 tbsp) water, then stir into the chocolate sauce and cook for 3 - 4 minutes.
5. Stir in the liqueur and the sugar.