Cream together the unsalted butter, caster sugar and light brown sugar and vanilla extract until they are thoroughly combined.
Add the egg and continue mixing, then add the egg yolk and do the same - try to add it slowly so that the mixture doesn't split!
Add the plain flour, bicarbonate of soda, and sea salt) and mix until smooth.
Fold through the milk chocolate chips.
Chill the mixture in the fridge overnight/24 hours, covered - this will allow it to firm up perfectly! I split my mixture into 3 batches, and roll in clingfilm into sausages, then when they're chilled you can just slice it - but its up to you!
Once the dough has been chilled, preheat the oven to 200ºC and line baking trays with parchment paper!
Carefully slice/roll the dough into 1-2cm thick rounds and spread equally onto the trays.
Sprinkle with a touch of sea salt and bake in the oven for 10-12 minutes until golden around the edge and cooked in the middle.
Leave to cool on the tray for 5 minutes, then transfer to a wire rack and continue to cool! - Enjoy!
Notes
You obviously don't have to use the sea salt on top of the cookies, but I like the difference of textures and the salt complements the chocolate well.
You can freeze the cookie dough in the rolls for up to 3 months (Mine never lasts more than a month) as I do double batches, and then bake them when I need/want them! They take a couple more minutes in the oven as they will be frozen - be careful when cutting them as it'll be extra hard!