Unwrap the chocolates, place in the freezer on a tray and pop. Heat the fan / gas furnace to 180C/160C 4. The base and sides of a 21-22 cm square tin with baking parchment are lightly greased and lined.
Put the butter, sugars and cocoa into your biggest casserole and melt softly together, stirring constantly to prevent the combination from catching. Once the sugar granules have just about disappeared, take off the heat, tip into a bowl and leave to cool for 5 mins.