Start by browning 120g of the butter. To do this, chop the butter into evenly sized pieces then place into a medium saucepan. Place over a medium heat & allow to melt, stirring regularly with a spatula. Keep cooking, until the butter starts to foam, smell nutty & you can see that the milk solids in the bottom of the pan have turned golden brown.
Once the butter has browned, transfer the butter to a bowl & leave to cool to room temperature. In the meantime, chop the remaining 80g of butter into small pieces & leave to soften at room temperature.The brown butter should take around 30 minutes to cool. Add both of the butters, the dark brown sugar, caster sugar & treacle into a mixing bowl then whisk until combined.
Next, add in the egg, yolks & vanilla then whisk again, to combine.
To the mixing bowl, add the flour, cornflour, baking powder, bicarbonate of soda & salt then fold together with a spatula until the flour is just incorporate.
Roughly chop 200 grams of the milk chocolate, stir into the cookie dough then chill in the fridge for 30 minutes.Chilling the cookie dough firms it up slightly, making it easier to scoop. Whilst the cookie dough is chilling, chop the remaining 50g of milk chocolate into 12 chunks.
Using a cookie scoop (or a spoon), portion the chilled cookie dough into 12 equally sized portions. Each should weigh around 85 grams.
With your hands, roll a portion of cookie dough into a ball then press a milk chocolate chunk into the top. Repeat with the remaining dough.
Place the balls of cookie dough into an airtight container & refrigerate overnight.