Step 1

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

Step 2

Add the egg and egg yolk, and mix until fully incorporated.

Step 3

Add the melted and cooled chocolate, and mix until combined.

Step 4

Sift together the gluten free flour blend, cocoa powder, xanthan gum, baking powder, baking soda and salt. Add the dry ingredients to the wet in two batches, mixing well after each addition. The final cookie dough will be soft and slightly sticky to the touch.

Step 5

Stir in most of the chocolate of choice, reserving a few tablespoons for later.

Step 6

Chill the dough in the fridge for 15 – 20 minutes until slightly firmed up but still scoopable.

Step 7

Adjust the oven rack to the middle position, pre-heat the oven to 355 ºF (180 ºC) and line a baking sheet with baking/greaseproof paper.

Step 8

Using a "2 tablespoon" ice cream or cookie scoop, shape the cookie dough into half spheres/balls. Arrange the cookies on the lined baking sheet so that they have enough space (about 2 inches/5 cm) around them for spreading out during baking – I usually bake 6 to 8 cookies at a time.

Step 9

Bake the cookies at 355 ºF (180 ºC) for 12 – 14 minutes, depending on your preferred cookie texture and consistency. (You can shorten the baking time to about 10 – 11 minutes for very gooey cookies, or prolong it to 16 – 17 minutes for very crisp cookies.)At the 8 minute mark, take the baking sheet out of the oven, tap/bang it 5 – 7 times onto the kitchen counter, and place the reserved chocolate on top of each cookie, then return to the oven. (Tapping the cookie sheet on the kitchen counter helps the cookies to flatten, thus giving them their characteristic shape. Omit the tapping if you want thicker cookies.)Return the cookies to the oven and bake until done. You can repeat the tapping/banging step once or twice more until the cookies are done.

Step 10

Immediately out of the oven, you can correct the shape of the cookies into a more evenly round one. To do this, use a round cookie cutter slightly larger than the cookie diameter to nudge the cookies into shape.

Step 11

Allow the cookies to cool on the baking sheet for 2 – 3 minutes, then transfer to a cooling rack to cool completely. Sprinkle the cookies with sea salt (optional).

Step 12

Storage:The double chocolate chip cookies keep well for about one week in a closed container in a cool dry place.