Lightly grease a baking tray (8"x12") and turn the oven to 190°C/170°C/gsmk5

Line the tray with baking/greaseproof paper

Melt the butter and chocolate together in a bain-marie (That's a bowl on top of a heated pan with water inside)

Take the bowl off the heat and stir in roughly 1/3 of the sugar to cool it slightly and then add small amounts of egg and the rest of the sugar alternately until all is mixed in

Gently fold in the sifted flour

Add the walnuts if using

Put mixture in the tin and bake for 20 mins