Preheat the oven to 180°C/350°F/Gas 4. Grease a large baking tray with butter.
In a large bowl, cream the butter, sugar, egg yolk and vanilla extract with a wooden spoon.
Add the flour gradually, stirring to make thick dough. Then stir in the chocolate chips.
Scoop out a dessertspoon of dough at a time, mould into a ball and place on the greased baking tray. Press with your fingertips to flatten into biscuit shapes. Leave plenty of space between each biscuit.
Bake for 12–15 minutes, until lightly golden but still a little soft in the middle. Let them cool a little before eating. Easier said than done.