Mix the margarine with the sugar in a large bowl.
Add vanilla extract and a pinch of salt, mix further.
Add the flour and mix until a fairly stiff dough is formed. You will have to use your hands at the end to knead the dough a little to ensure it is mixed well
Chop your chocolate into smallish chips, then mix these in well by hand.
Roll into a large ball, place in a bowl, cover and put in the fridge to chill for at least an hour.
Once chilled, roll out and cut into circles. Put on a baking tray covered with greaseproof paper.
Bake in the oven at 200 degrees cesius (fan) for around 30 - 40 minutes. My oven seems to cook quite slowly, so ensure you check regularly. The edge will begin to brown, if the shortbread isn't cooked at this point turn down the temperature and cook a little longer.
I allowed the shortbread to cook on the tray before moving.