Dissolve 125 g (4 oz) sugar in the water, occasionally stirring. When dissolved, increase the heat and boil for 5 minutes without stirring until golden brown, keeping a watchful eye towards the end of the cooking time.
Mix the cocoa in a small bowl with 2 tablespoons of the boiling water. Mix the remaining sugar with the eggs, egg yolks and biscuits in a second larger bowl.
Take the syrup off the heat as soon as it caramelizes. Add the remaining boiling water and tilt to mix. Pour into a 900 ml (1½ pint) ovenproof china dish. Tilt to coat the base and halfway up the sides. Stand in a roasting tin.
Pour the milk into the drained caramel pan and bring just to the boil. Stir the cocoa mix into the egg mixture, then gradually whisk in the hot milk, then the coffee. Slowly pour into the caramel-lined dish.
Fill the tin with hot water halfway up the sides of the dish, then cook in a preheated oven, 160°C (325°F), Gas Mark 3, for 50–60 minutes until the custard has just set but still wobbles slightly in the centre.
Take dish out of tin, leave to cool, then chill for 4–5 hours or overnight. To turn out, stand in just-boiled water for 10 seconds, then invert on a plate with a rim. Decorate with chocolate curls or crumbled biscuits.