Line a cake tin with greaseproof paper that fits in the freezer
Add the softened butter and sugars to your mixing bowl and mix until mixed well but not smooth and pale as this will make the cookies spongy and cake like
Mix flour, sea salt and bicarb together giving it a quick dry whisk, set aside
Add the egg yolks and vanilla extract to the butter/sugar and mix well
Now add the dry ingredients followed by the chopped chocolate
With all the ingredients thoroughly mixed, it's time to make the cookies. I use an ice cream spoon to make little balls, but you can use any utensil you prefer. Make little balls of roughly of 60-70 grams and you should get about 16 cookie balls
Put the balls in to the prepared cake tin and leave in the freezer for at least an hour covered with cling film. This step firm up the cookie dough, so that they don't spread too much when cooking, giving them a soft center and crispy edge. You can leave them in the freezer for up to a month. If you do just remember to leave them out 15 mins before cooking to slightly defrost
As they spread during cook time I use two baking sheets, giving them plenty of space
Bake for about 18 mins
When done they should have a soft center and slightly darkened crispy edge. Leave them to cool on the baking tray for at least 10 mins before transferring on to a wire rack to cool completely