Cream together the margarine, sugars and salt.

Gently mix in the flour, bicarbonate and cocoa and combine to form a soft dough.

Stir in the chopped chocolate, making it evenly distributed.

Form into a sausage shape and wrap in baking paper. Place in the fridge to chill, you want it to be as cold as possible

Preheat the oven to 170 degrees centigrade

Line two baking sheets with parchment. Slice the cookie dough into 1cm slices and place well apart on the baking sheets

Bake for 11-12 minutes. They should have spread out nicely.

Cool briefly on the sheets so they are stiff enough to move and then transfer to a wire rack.