175g unsalted butter
325g caster sugar
130g plain flour
Preheat the oven to 150°C. Grease and line a baking tray.
Put the chocolate and butter in a heatproof bowl over a saucepan of simmering water (don't let the base of the bowl touch the water). Leave until melted and smooth.
Remove from the heat. Add the sugar and stir it. Add the flour and stir it. Finally, stir in the eggs and mix until smooth.
Spoon the mixture into the tray. Bake for about 35 minutes, until the top is flaky and the middle is soft.
Leave to cool before slicing.