Preheat oven to 180°C/350°F/Gas Mark 4. Line the base and sides of a 20 cm (8 in) springform cake tin with baking paper. Melt chocolate and butter in a metal bowl over a pan of simmering water. Separate eggs, and beat egg whites until stiff with 60 g (2 oz) of the sugar. Beat yolks with remaining sugar, then mix this thoroughly into chocolate. Gently fold a third of the whites into chocolate mixture, then gently fold in the remainder.
Pour into prepared tin and bake in the middle of the oven for 25–30 minutes. It’s ready when a crust has formed, but the cake must be wobbly – if not, it’ll be overcooked and set too hard.
Remove from the oven and cover tin tightly with foil – the steam keeps the heat in and softens the crust. Once cold, chill for at least 4 hours before removing from the tin and spreading the ganache on top. To cut, heat a sharp knife in a jug of hot water – it will slice easier.