Heat the fan / gas furnace to 180C/160C 4. Butter a 30x 20 cm non-stick tin and a non-stick baking parchment form the base.
Fill a tiny casserole with water, take to a simmer and placed on top of it a snug-fitting heat-proof bowl. Add butter and chocolate and melt softly and stir occasionally over low heat. Be cautious not to allow it on the bottom to catch and burn. Remove from the heat and leave to cool a little.