Preheat oven to 350°F (175°C). Grease a 9-inch springform pan and line bottom with parchment. Melt 8 oz chocolate and 1/2 cup butter together over a double boiler or in the microwave, stirring until smooth. Let cool slightly.

Beat 4 eggs, 3/4 cup sugar, 1 tsp vanilla, 1/4 tsp salt, and 1/2 tsp espresso powder on high speed until thick, pale, and tripled in volume, about 5 minutes.

Fold the cooled chocolate mixture gently into the egg mixture with a spatula until just combined and uniform in color.

Pour batter into the prepared pan. Bake for 25 to 30 minutes, until puffed and crackly on top, with a slight jiggle in the center. A toothpick will have moist crumbs.

Cool in pan for 15 minutes, then run a knife around the edge, release the springform, and invert onto a plate. Remove parchment, flip right-side up, and dust with 1 tbsp cocoa powder. Serve warm or room temperature.