Add the unsalted butter to a saucepan (preferably one with a light-coloured interior that allows you to see the butter changing colour) and cook it over medium heat with frequent stirring for 6-8 minutes in total.The butter will first melt and then start bubbling and foaming. Finally, it will turn amber and smell nutty. You should see specks of a deep amber colour on the bottom of the saucepan (those are the caramelised milk solids).Be careful not to overcook and burn your butter – remove the brown butter from the heat as soon as the milk solids turn an amber colour.

Pour the browned butter into a large bowl (or the bowl of a stand mixer, if using), and allow it to cool until it firms up and thickens slightly: it should have the texture of very soft, spreadable butter. (You can speed up the cooling process by placing the bowl into a cold water bath.)

You can prepare the cookie dough using a stand mixer fitted with the paddle attachment or a hand mixer fitted with the double beaters.

Add the light brown sugar, caster/superfine or granulated sugar, yoghurt and vanilla to the cooled and thickened brown butter, and cream them together until pale and fluffy.

In a separate bowl, whisk together the dry ingredients: gluten free flour blend, cornstarch (US)/cornflour (UK), xanthan gum, baking powder, baking soda and salt.

Add the dry ingredients to the brown butter-sugar mixture and mix well until you get a smooth, fairly soft cookie dough.

Add in most of the chopped chocolate and chocolate chips (reserving some for adding on top of the cookie dough balls before baking) and mix well until evenly distributed.

Chill the cookie dough in the fridge for at least 2 hours or ideally overnight (in a bowl covered tightly with plastic wrap/cling film).Tip: If you’re impatient, you can bake the cookies after only a 2 hour chill in the fridge – but I definitely recommend chilling them overnight. The longer chilling time greatly improves their texture and flavour.

Adjust the oven rack to the middle position, pre-heat the oven to 350ºF (180ºC) and line two large baking sheets with parchment/baking paper.Tip: You'll bake the cookies in two batches, so you can line two baking sheets if you have them on hand. Otherwise, just re-use the same baking sheet, but make sure to cool it completely before you place the next batch of unbaked cookies onto it.

Portion out the chilled cookie dough into eight large 100g (3.5oz, about 4½-5 tablespoons) cookie dough balls.Tip: You can make the cookies smaller or larger if you wish, just keep in mind that smaller cookies will need less time in the oven while larger cookies will require a slightly longer baking time.

Place the cookie dough balls on the lined baking sheets, making sure that they're as much spaced out as possible (as they're large cookies and they'll spread quite a bit during baking). I usually bake 4 cookies per baking sheet.Press the reserved chopped chocolate and chocolate chips on top of the cookie dough balls.

Bake the cookies, one baking sheet at a time, at 350ºF (180ºC) for 14-16 minutes or until they’re light golden on top with slightly darker edges and the centres are still soft and noticeably underdone.While the first baking sheet is in the oven, keep the other one in the fridge.Tip: Keep in mind that the cookies will continue baking for a couple of minutes after you’ve taken them out of the oven (due to the heat retained by the baking sheet and carryover cooking) – so those underdone centres will result in the absolutely gooey texture.

Immediately out of the oven, while the cookies are still very hot, soft and malleable, you can use a round cookie cutter (larger than the cookie diameter) to nudge or scoot the cookies into a more perfectly round shape.

Allow the cookies to cool on the baking sheet for about 10 minutes, then transfer them to a cooling rack. Enjoy them warm or cooled completely to room temperature.

The brown butter chocolate chip cookies keep well in a closed container at room temperature for about 1 week. Note that the cookies will get a bit softer and chewier with time.Alternatively, you can store raw, unbaked balls of cookie dough in an airtight container in the fridge for about 1 week, or in the freezer for 1-2 months. Bake the cookie balls directly from chilled or frozen, just adding 2-3 minutes to the baking time to compensate. Note that the longer the cookie dough balls are in the fridge or freezer, the less they will spread during baking (so they'll end up thicker).