Prepare: Preheat the oven to 160°C fan (320°F) and line a large baking tray with baking paper.
Melt the Chocolate: Place the dark chocolate and butter in a heatproof bowl over a pan of gently simmering water, making sure the bowl doesn't touch the water. Stir occasionally until completely melted and smooth, then remove from the heat and leave to cool slightly.
Whip the Eggs and Sugars: Add the eggs, caster sugar and light brown sugar to a large bowl. Beat with an electric hand whisk on high speed until pale, thick and fluffy.
Add the Chocolate: Pour the slightly cooled chocolate and butter mixture into the whipped eggs and sugars. Whisk gently until completely combined.
Add the Dry Ingredients: Sift in the cocoa powder, flour, baking powder and salt. Gently fold until just combined and no dry streaks remain. The mixture will be wet and sticky rather than firm like traditional cookie dough.
Scoop the Biscuits: Portion the mixture onto the prepared tray using an ice cream scoop or large spoon, leaving plenty of space between each biscuit because they will spread as they bake.
Bake: Sprinkle with flaky sea salt, if using, and bake for about 12 minutes. The biscuits are ready when the tops are crackled and the edges are set but the centres still look soft.
Cool: Leave the biscuits on the baking tray for 10 minutes before carefully transferring them to a wire rack. They will be very soft when they first come out of the oven and will continue to set as they cool.