Preheat the oven to 190ºC/170ºC Fan and Line 2 Large baking trays with parchment paper.
Melt the butter slowly until it is fully melted, leave to cool for a couple of minutes.
Once cooled, beat in both the light brown soft sugar and granulated sugar until they are all thoroughly combined.
Add the egg, egg yolk and vanilla extract and beat the mixture until the mixture becomes much lighter in colour, is smooth, and has increased in volume.
Add the dry ingredients (plain flour, bicarbonate of soda, and sea salt) and beat again until smooth.
Fold through the chocolate chips.
Using a 5cm Scoop, portion the cookies onto the trays (I did 6 per tray).
bake the cookies in the oven for 12-13 minutes for softer chewy cookies, OR 15 minutes for soft/crunchy cookies.
Leave the cookies on the tray for 5 minutes after they have finished baking, and then transfer to a wire rack and continue to cool! Enjoy!
Notes
You can chill this dough in the fridge to get a softer texture, but if you prefer chewier cookies than this is the way!
You can also freeze the raw cookie dough in balls for up to 3 months.
If you don't beat the the butter/sugar/egg mixture enough to the texture I have described, then the cookies will flatten into pancakes! Its very important to beat the mixture so its lighter than the original mixture, if you have done it correctly, once you have added the flour, it'll be VERY thick and the chocolate chips will be reasonably difficult to fold in!
I would also say that you MUST use real butter and not margarine in these cookies so that the results are best.