Grease and line a 28 x 18 cm (11 x 7 inch) baking tin.
Place the chocolate and butter in a large heatproof bowl over a saucepan of simmering water and leave until melted. Stir in all the remaining ingredients and combine well.
Pour the mixture into the prepared tin and place in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes until slightly springy in the centre.
Remove from the oven and leave to cool for 10 minutes in the tin, then cut into 15 squares. Serve with a generous dollop of vanilla ice cream.