Preheat your oven to 160°C (320°F) with a fan or 170°C (338°F) for a conventional setting.

Melt chopped dark chocolate and butter in a heatproof bowl over gently simmering water, stirring until smooth (about 5 minutes).

Beat eggs, caster sugar, and demerara sugar in a separate bowl until pale, thick, and fluffy (about 5 minutes).

Sift together all-purpose flour, cocoa powder, baking powder, and salt in a mixing bowl. Stir in chocolate chips.

Fold the melted chocolate mixture into the whipped egg and sugar mixture, then gradually incorporate the dry ingredient mixture until just combined.

Drop generous spoonfuls of the batter onto a lined baking tray, spaced adequately. Bake for 12-14 minutes until set and lightly crackled on top.

Sprinkle with sea salt, cool on the tray for about 10 minutes, then transfer to a wire rack.