Using a stand mixer fitted with the paddle attachment or a hand mixer fitted with the double beaters, cream the butter and both sugars together until pale and fluffy.

Add the egg, egg yolk and vanilla bean paste, and mix until fully combined.

Sift together the gluten free flour blend, xanthan gum, baking powder, baking soda and salt. Add the dry ingredients to the wet in two batches, mixing well after each addition.

Add in the chocolate chips and half of the chopped chocolate, and mix well until evenly distributed.

Chill the cookie dough in the fridge for about 1 hour until firm enough to scoop and handle easily.

Use a '2 tablespoon' ice cream or cookie scoop to scoop individual portions of cookie dough. You should get 16 cookies in total.

Place them onto a baking sheet lined with baking/greaseproof paper, cover tightly with cling film and chill in the fridge for at least 30 minutes.

Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line another baking sheet with baking/greaseproof paper.

In batches (at most, 6-8 at a time), place the chilled cookie dough balls on the lined baking sheet, leaving plenty of space between them.

Bake the cookies at 350ºF (180ºC) for 12-15 minutes in total, depending on your preferred cookie texture and consistency.

At the 8-9 minute mark, take the baking sheet out of the oven, tap/bang it 5-7 times onto the kitchen counter, and place a few pieces of the reserved chocolate on top of each cookie.

Return the cookies to the oven and bake until done.

Immediately out of the oven, you can correct the shape of the cookies into a more evenly round one.

Allow the cookies to cool on the baking sheet for about 5 minutes, then transfer to a cooling rack to cool completely.

Once all the cookies are baked and cooled, sprinkle with flaky sea salt (optional) before serving.