575g caster sugar
675g best-quality dark chocolate (70% cocoa solids), broken into small pieces
450g unsalted butter, softened
Preheat the oven to 130°C. Use greaseproof paper to line the base of a 30cm round cake tin.
Beat the eggs with a third of the sugar in a free-standing electric mixer until the volume quadruples – this will take at least 10 minutes.
Meanwhile, melt the chocolate and butter together in a heatproof bowl set over a pan of boiling water. The water should not touch the base of the bowl. Remove from the heat.
Heat the remaining sugar in a small pan with 220ml water until the sugar has completely dissolved to a syrup, stirring occasionally. Gently pour the syrup into the melted chocolate, stirring.
Reduce the speed of the mixer and slowly add the warm chocolate and syrup mixture to the eggs. Increase the speed and continue beating until completely combined. The mixture will lose volume.
Pour into the cake tin and place in a bain-marie (bath of hot water). For the cake to cook evenly, it is essential that the water level comes at least two-thirds up the side of the tin. Bake for 75 minutes or until set.
Remove the cake tin from the water. Leave the cake in the tin to go completely cold before turning out (don’t refrigerate it).