Heat 150 g (5 oz) sugar gently in a pan with the measured water until dissolved into a light syrup. Remove from the heat.
Melt the chocolate with the butter in a bowl over a pan of simmering water (see
Whisk the rest of the sugar together with the eggs until they are light and foamy and tripled in volume. Beat the chocolate into the whisked eggs slowly until well mixed in.
Grease a 23 cm (9 inch) springform cake tin and line with nonstick baking paper. Tip the mixture into the tin and rest the tin on a trivet in a roasting tin. Three-quarters fill the roasting tin with boiling water. Bake in a preheated oven, 120°C (250°F), Gas Mark ½, for 50 minutes until just firm.
Leave the cake to cool in the tin in the water. Serve in wedges with a few blueberries and a drizzle of crème de cassis.