Preheat the oven to 180°C. In a large bowl, cream together the butter and sugars. Add the egg yolk and vanilla extract, and mix. Stir in the flour until well combined and finally add in the chocolate chips, so they are evenly distributed. press the dough into the moulds you pick. If you cant find anything that is cup like and is suitable for the oven, you can use dariole moulds. You want the walls of the cup to be about 1 centimeter thick. Line the inside of the cups and fill with baking beans or a suitable baking weight. Chill for 30 minutes in the fridge.
Bake for about 14 minutes or until the cookies just start to brown, before removing the baking beans and baking for a further 5 minutes or until the cookies are cooked through. Remove from the oven, and let cool completely. Pour the melted chocolate around the inside of the cups till completely coated, removing any excess chocolate and then place in the fridge until set.