Preheat your oven to 350°F (175°C) and position the rack in the center. Line your pan with parchment paper and allow a slight overhang.

Set a heatproof bowl over simmering water, add chopped butter and dark chocolate, and stir for 4–5 minutes until melted and smooth. Remove from heat and let cool for 2 minutes.

Stir granulated sugar into the warm chocolate mixture until combined. Crack in eggs one at a time, whisking after each, then whisk in vanilla until glossy.

In a separate bowl, whisk together flour, cocoa powder, baking powder, and kosher salt, breaking up any lumps.

Fold the dry ingredients into the chocolate mixture gently with a spatula until no streaks remain, taking care not to overmix.

Pour hot water around the edges of the batter and fold in with a J-shaped motion until just combined and slightly thinned.

Transfer the batter to the prepared pan, smooth the top, and gently tap the pan on the counter three times to remove air bubbles.

Bake for 32–36 minutes, until the top is matte and set and a toothpick inserted in the center comes out with moist crumbs.

Cool in the pan on a wire rack for 10 minutes, then lift out using the parchment overhang and cool completely for at least 30 minutes before slicing.