Pre-heat the oven to 180 C / Gas 4. Line a baking tray with greaseproof paper.
In a large bowl, cream together the softened butter and sugar. Add in the beaten egg and the vanilla extract, and mix well. Sift in the flour and stir to a thick but sticky dough then add the dark chocolate chips.
Roll into small, evenly sized balls and pat down slightly on the prepared baking tray to flatten, be sure to spread them quite far apart so they don't stick together.
Bake in the preheated oven for around 10 minutes, until golden brown. Let them cool on the baking tray for another 10 minutes and enjoy.
If you prefer, you can substitute the dark chocolate chips for milk or white chocolate or any kind of nuts or dried fruit.