In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, dark muscovado sugar, granulated sugar, and salt. Beat on medium-high speed until the mixture is light and fluffy, about 3 mins, pausing to scrape down the sides as needed.

Add the egg and beat until it has been fully incorporated, scraping down the sides to make sure all the butter is also incorporated.

In a medium bowl, whisk together the all-purpose flour, cornstarch, baking soda, and baking powder.

Place a sieve over the stand mixer bowl and sift the dry ingredients into the wet ingredients. Mix on low speed just until the flour has been incorporated.

Add the chopped dark chocolate and mix again to incorporate.

Roll the dough into 65g balls in your hand and place them on a baking tray. Cover with cling film and refrigerate for 2 hours.

Preheat the oven to 170°C.

Working in batches, place 6 dough balls on a baking tray lined with parchment paper. Press a few more bits of chopped dark chocolate into the top of the balls.

Place the tray in the oven and bake for 12 to 14 mins or until lightly golden.

Remove the tray from the oven and while still hot, take a cookie cutter slightly larger than the baked cookie and quickly rotate it around the cookie. This neatens up the edges and gives you a perfectly round cookie.

Allow the cookies to cool on the tray for 5 mins before transferring them to a cooling rack to completely cool.