675 g (1.1/2 lb) bitter-sweet chocolate, broken into small pieces
10 whole eggs
575 g (1 lb 5 oz) caster sugar
450 g (1 lb) unsalted butter, softened
For ten to twelve
Preheat the oven to 160°C/325°F/Gas 3.
Line a 30 × 5 cm (12 × 2 in) cake tin with greaseproof paper, then grease and flour it.
Beat the eggs with one-third of the sugar until the volume quadruples – this will take at least 10 minutes with an electric mixer.
Heat the remaining sugar in a small pan with 250 ml (8 fl oz) water until the sugar has dissolved to a syrup.
Place the chocolate and butter in the hot syrup and stir to combine. Remove from the heat and allow to cool slightly.
Add the warm chocolate syrup to the eggs and continue to beat, rather more gently, until completely combined – about 20 seconds, no more. Pour into the cake tin and place in a bain-marie of hot water. It is essential, if the cake is to cook evenly, that the water is hot and that it comes up to the rim of the tin. Bake in the preheated oven for 40 minutes to 1 hour or until set. Test by placing the flat of your hand gently on the surface. The cake should feel just set not firm like a baked cake.
Leave to become completely cold in the tin before turning out.