Preheat the oven to 180°C and grease a 30cm x 20cm tin.
Put butter and cocoa into a big saucepan and melt gently, DO NOT BOIL. Add Chelsea Soft Brown Sugar and vanilla and stir well. Take off heat and sift in flour.
Add eggs and beat really well (this is easy enough to do by hand). Add in the chopped walnuts (optional). Spread evenly in tin and bake for 20-25 minutes.
When cool, you can ice with a thick chocolate icing but by no means does it need it!