Set the oven to 180°C or 160°F with the fan on. Use baking paper to line 2 baking trays. Butter and sugars should be blended well and thoroughly beaten for 1-2 minutes, either with an electric mixer or by hand. Add egg and vanilla after blending.
Add flour and mix. Add the milk and dark chocolate chunks. Place 2 level tablespoonfuls of the mixture, rolled into balls, 3 cm apart on the prepared trays. Push down just a little. Add extra chocolate chunks for decoration.
Bake for 15 to 18 minutes, or until done and light brown. To cool, transfer to a wire rack. For up to a week, store in an airtight container.