Preheat oven to 170°C.
Beat oil and sugar and stir in the flour.
Melt chocolate in double boiler or bain-marie and add to dough.
Mix well to form a soft dough. Put in the fridge to cool for 30 minutes.
Pinch off small portions and form into small round balls.
Place on cookie sheet lined with wax paper, leaving ample space between them.
Press a half walnut in center of each cookie.
Bake in moderate oven for about 12-15 minutes.