Melt the milk chocolate (100g). This can either be done in short bursts in the microwave (no more than 20 seconds each time, stirring in between), over a bain-marie, or in a saucepan over a VERY low heat. Once melted set aside to cool.
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 (260g plain flour, 40g cocoa powder, 1½ tsp bicarbonate of soda, ⅛tsp salt) and mix until fully combined.
Add the melted milk chocolate and fold into the mixture until it's combined.
Finally mix in the chocolate chunks (200g white chocolate, 50g dark chocolate) 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. Refrigerate 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 on 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 crisp around the edges but still soft 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!