Preheat oven to 180°C (160°C fan) and line baking trays with parchment paper.
Melt chocolate, sugar(for chocolate mixture) and butter in a heatproof bowl over simmering water. Stir until smooth, then set aside.
Beat sugar(for the egg), and egg with a mixer on medium-high for 5 minutes until pale and fluffy.
Add melted chocolate to the egg mixture and beat on medium-low for 1 minute.
Add flour, cocoa, baking powder, and salt to wet ingredients and stir until just combined.
Scoop dough onto trays, spacing well apart. Optionally, sprinkle with sea salt and bake straight away because if you rest the batter you will not get the crinkly tops.
Bake for 10 minutes. Cool on trays for 10 minutes before transferring the cookies
