Preheat your oven to 160°C (320°F) for a fan oven or 170°C (338°F) for conventional baking. Line a baking tray with parchment paper.

Melt chocolate and butter together in a heatproof bowl over simmering water. Stir until smooth, then mix in vanilla extract.

In a large bowl, beat together egg, caster sugar, and demerara sugar for about 5 minutes until thick and fluffy.

Sift the plain flour, cocoa powder, baking powder, and salt together. Fold in chocolate chips if using.

Gently fold the melted chocolate mixture into the whipped egg mixture, then gradually add dry ingredients until just combined.

Scoop the batter onto the prepared baking tray and space them apart. Bake for 12–14 minutes.

After baking, rest brookies on the tray for about 10 minutes, then transfer to wire rack to cool completely.