Preheat the oven to 160ºC (320ºF) fan or 170ºC (338ºF) conventional. Line a baking tray with parchment paper.
In a heat-safe bowl, melt together the chopped dark chocolate and butter over a pan of simmering water, ensuring the bowl does not touch the water. Stir occasionally until smooth, then mix in the vanilla.
In a separate medium bowl, use a hand-held electric whisk on high speed to beat the caster sugar, demerara sugar, and eggs. Continue beating until the mixture becomes pale, thick, and voluminous.
In another bowl, sift together the flour, cocoa powder, baking powder, salt, and chocolate chips to ensure an even mixture.
Carefully fold the melted chocolate mixture into the whipped egg and sugar blend, taking care to retain the airiness.
Gently incorporate the dry ingredients into the wet mixture using a spatula, mixing only until combined.
Portion the batter using a tablespoon onto the prepared baking sheet, leaving space between each scoop for spreading. Bake for 12–14 minutes, or until the tops appear set and crackly.
Upon removal from the oven, optionally sprinkle with sea salt. Allow the brookies to cool on the tray for 10 minutes before transferring them to a wire rack.