Beat together the butter (200g) and sugars (90g soft dark brown sugar, 110g caster sugar) until fully combined and lighter in colour (this should take 2-3 minutes with an electric mixer).
Add the egg (1 whole large egg + 1 yolk) and the vanilla extract (1 tbsp) and beat until fully combined.
Sift in the dry ingredients (300g plain flour, 1½ tsp bicarbonate of soda, ⅛ salt) and mix together until fully combined.
Finally mix in the chocolate chunks (250g) until they are evenly distributed in the mixture.
Roll the dough into a sausage shape about 5cm in diameter (I find it easiest to split the dough into two and make two sausages) and wrap in cling film. You may find it easier to use the clingfilm to help you get an even sausage shape as the dough will be a little sticky. Refrigrerate for at least 1 hour (up to 5 days).
When you're ready to bake your cookies, preheat your oven to 190°C/170°C fan. Remove your cookie dough from the fridge and using a sharp knife slice into round cookies about 1½cm thick. Place them onto a baking sheet (lined with greaseproof paper or a non-stick baking sheet). Leave space between each one as they'll double in size as they cook. Cook for 8-9 minutes until golden and crisp around the edges but still pale in the middle.
Once cooked, remove them from the oven. Leave them to cool for a few minutes on the tray before moving them to a wire rack to cool completely.
Enjoy!