Preheat the oven to 175°C (350°F). Line two heavy-gauge baking sheets with silicone mats or parchment paper.

In a heatproof bowl set over a saucepan of barely simmering water (double boiler), melt the chopped bittersweet chocolate and butter together. Stir until glossy and completely smooth. Remove from heat and allow to cool to approximately 35°C (95°F).

In a stand mixer fitted with the whisk attachment, combine the eggs, granulated sugar, and light brown sugar. Whisk on medium-high speed for exactly 5 minutes. The mixture must reach the 'ribbon stage'—it should be pale, thick, and tripled in volume.

With the mixer on low speed, slowly stream in the melted chocolate-butter mixture and vanilla extract. Mix until just incorporated.

Sift the flour, cocoa powder, baking powder, and fine sea salt directly into the bowl. Using a silicone spatula, gently fold the dry ingredients into the batter until no streaks of flour remain.

Fold in the semi-sweet chocolate chips. Do not overmix, as this will result in a cakey rather than fudgy texture.

Using a 40mm (1.5 tablespoon) cookie scoop, immediately drop portions of dough onto the prepared baking sheets, spaced 5cm (2 inches) apart. Do not refrigerate the dough; immediate baking is required for the crackly top.

Bake for 10 to 12 minutes. The cookies are finished when the edges are set but the centers still appear slightly soft and the tops have formed a shiny, cracked crust.

Remove from the oven and immediately sprinkle with flaky sea salt. Allow the cookies to cool on the baking sheet for at least 15 minutes to allow the internal structure to set before transferring to a wire rack.