This dish is delicious served with vanilla or pistachio ice cream.
Preheat the oven to 180C/350F/Gas 4. Grease a large baking mould.
In a small bowl resting over a saucepan of hot water, melt the chocolate and butter together (the bowl should not be touching the water).
In a separate bowl whisk the egg yolks with the icing sugar until pale and creamy.
Fold the melted chocolate into the egg mixture.
Using a clean whisk, whisk the egg whites until soft peaks form, then fold gradually into the chocolate mixture.
Using a spatula pour the mixture into the greased mould, then bake in the oven for approximately 30 minutes, until slightly cracked on the top, but still gooey in the middle. Serve.