In a mixing bowl, cream the sugar, margarine and the vanilla extract together with a folk until fluffy.
Fold in flour and salt in parts until well combined. Stir in the chocolate and mixed nuts and form the dough into a cylinder shape (about 4.5cm in diameter). The dough should be very soft but not too wet.
Wrap the dough tightly in cling film and leave in the freezer for one hour.
Preheat the oven to 180°C (gas mark 5) and line a baking tray with baking paper. Take out the dough from the freezer, unwrap and cut into slices with a bread knife (thickness of between 0.5cm to 0.7cm). You will have 20 to 25 cookies.
Place the cookies on the prepared baking tray and bake for about 16 minutes of until slightly golden around the edge. Leave to cool before serving. The cookies may be soft when they just come out from the oven but they will go crunchy after cooled down.
Keep in an air tight container up to 5 days in room temperature.