Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.

Melt the chopped dark chocolate and butter together in a double boiler or microwave (30 second bursts). Stir until smooth and let cool slightly.

In a large bowl, beat the eggs, granulated sugar, and brown sugar on medium-high speed for 5 minutes until pale and doubled in volume.

Pour the cooled chocolate mixture and vanilla into the egg mixture. Mix on low speed just until combined.

Sift the flour, cocoa powder, baking powder, and fine salt into the bowl. Gently fold with a spatula until no flour streaks remain.

Use a cookie scoop to drop batter onto baking sheets, spacing them 2 inches apart.

Bake for 10-12 minutes. The cookies should have shiny, crackly tops but still look soft in the center.

Remove from oven and immediately sprinkle with flaky sea salt. Let cool on the pan for 10 minutes before moving to a wire rack.