In a stand mixer with the paddle attachment, or a large bowl with a wooden spoon, cream together the butter and the sugars. Add in the whole egg, the extra egg yolk, and the vanilla paste, and mix thoroughly until smooth.
Add in the dry ingredients - flour, baking powder, bicarbonate of soda, and 1/2 tsp of the sea salt (keeping aside the other 1/2 tsp to sprinkle on top of the cookies later) - and mix well to combine.
Fold in 100g of the chopped dark chocolate until evenly distributed. Cover the bowl and chill in the fridge for 90 minutes.
10 minutes before baking, line 2 large baking tray with baking paper or silicone, and preheat your oven to 170 °C
Divide the mixture into 15 cookies. An ice cream scoop is useful for this, or a couple of dessert spoons. Roughly shape into golf balls and spread out on the lined trays. Make sure you leave space, as the cookies need room as they cook.
Bake in the oven for 8 mins, remove them and dot the remaining 20g of chocolate chunks on top of the cookies along with a tiny sprinkling of sea salt. Put them back in the oven, switching the trays around from top to bottom, for the final 4 mins.
Remove the trays and leave to cool for 5 minutes. Transfer the cookies to a wire rack to fully cool. Although, I like to eat them slightly warm with the chocolate still all gooey!
Notes
You can make these sea salt chocolate chip cookies gluten-free simply by swapping out the plain flour for a gluten-free plain blend, and adding 1/2 tsp xanthan gum along with the dry ingredients.
I have made 15 cookies with this recipe. But it's up to you on the size of the cookies. You can be generous, and the mix will make 10 big cookies, or stretch it up to 20.
If you wish to freeze half a batch, to have fresh cookies another day, you can do so at the end of step 6. Open freeze the cookie dough balls (ie. spread them out on a tray and place in the freezer not touching each other). Once frozen, put them in a bag/container and seal well for up to 3 months. Defrost at room temperature for 30 minutes on a baking tray and follow the rest of the instructions.
You can turn these into Easter cookies very simply. Use 100g of chopped dark chocolate to put into the cookie mix. Then, instead of topping the cookies with 20g more chopped chocolate, take a 75g packet of Cadbury's Mini Eggs and bash the eggs lightly with a rolling pin to break them in half. After the cookies have been in the oven for 8 mins, remove and gently push the mini egg halves into the tops of the cookies (shell side up). Put back in the oven for a final 4 minutes. And there you have it! Easter egg chocolate chip cookies :-)