Flourless chocolate cake

225g/8oz good-quality dark chocolate, chopped into pieces
100g/3½oz unsalted butter, plus extra for greasing
4 large free-range eggs, separated
175g/6½oz icing sugar, sifted

1. Preheat the oven to 180C/350F/Gas 4. Grease a large baking mould.
2. 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).
3. In a separate bowl whisk the egg yolks with the icing sugar until pale and creamy.
4. Fold the melted chocolate into the egg mixture.
5. Using a clean whisk, whisk the egg whites until soft peaks form, then fold gradually into the chocolate mixture.
6. 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.