01 - Preheat the oven to 320°F with fan setting, or 338°F for conventional. Line a large baking tray with parchment paper.
02 - Place chopped dark chocolate and butter in a heatproof bowl over simmering water, ensuring the bowl does not touch the water. Stir occasionally until smooth, then remove from heat and mix in vanilla extract.
03 - In a medium bowl, combine eggs, caster sugar, and demerara sugar. Use an electric hand mixer on high speed to beat until the mixture is pale, thick, and forms ribbons.
04 - In a separate bowl, sift together flour, cocoa powder, baking powder, chocolate chips, and salt. Stir to combine evenly.
05 - Gently fold the melted chocolate mixture into the whipped egg and sugar mixture, maintaining the volume.
06 - Carefully fold the dry ingredients into the chocolate mixture using a spatula, mixing until just combined for a light batter.
07 - Drop tablespoons of batter onto the prepared tray, spacing them to allow spreading. Bake for 12–14 minutes, or until the tops are crackly and set.
08 - Remove from the oven and, if desired, sprinkle with sea salt. Cool on the tray for 10 minutes before transferring to a wire rack until completely cooled.