Light oven in advance - around 180 degrees – with my fan oven 150 is high enough.
Melt together in a saucepan one pack of butter (250gr) and one 200gr bar of dark (70% or more) chocolate (or two 100gr).
When they are melted take off the heat and mix in:
two-and-a-half to three mugs of sugar (coffee mug filled up to about an inch below the top),
one teaspoon salt,
just under 2 mugs plain flour,
nuts ( I don't usually put them in, but if you like them, pecans or walnuts are good),
two teaspoons cinnamon and three teaspoons vanilla essence.
Stir all together until well mixed.
Grease a roasting (or shallower) tin with butter or marg. Even better, line it with silicone paper. Pour in the mixture. Cook for about 20 minutes. I actually often cook them for slightly less than this. You may find they need more but they are a disaster if overcooked. They should be firm on the top but still gooey in the middle - experiment with a knife. Cut into squares while they are hot and then leave to cool before digging out.