Preheat your oven to 160ºC (320ºF) for a fan oven or 170ºC (338ºF) for a conventional oven. Prepare a baking tray by lining it with parchment paper.

In a heatproof bowl, combine the chopped dark chocolate and butter over a pan of simmering water. Stir often until melted and smooth, then add the vanilla extract.

In another bowl, beat the caster sugar, demerara sugar, and eggs together on high until thick and fluffy—about 5 minutes.

Sift together the flour, cocoa powder, baking powder, and salt in a separate bowl. Toss in the chocolate chips to prevent clumping.

Gently fold the melted chocolate and butter mixture into the whipped eggs and sugars until just combined.

Carefully add the dry ingredients to the wet mixture using a spatula, folding until no flour streaks remain.

Scoop tablespoon-sized portions of batter onto your lined tray and bake for 12-14 minutes. They should appear crackly and just set.

Let cool on the tray for about 10 minutes before transferring to a cooling rack.