Measure 100 g (4 oz) castor sugar and 1.5 dl (½ pint) water into a saucepan and stir over low heat to dissolve the sugar. Bring up to the boil and simmer for 1 minute. Then dip in 50 g (2 oz) cocoa powder. Whisk until the sauce is smooth and bring back to the boil. Draw off the heat and pour into a basin. Stir occasionally; the sauce will thicken up as it cools. Serve warm or cold.