Step 1
Chill soufflé dish. Cook ½ cup (1 stick) unsalted butter in a small skillet over medium heat, swirling occasionally, until it foams, then browns, about 4 minutes. Pour into a small heatproof bowl and chill until spreadable, about 20 minutes (butter should be opaque but still soft enough to brush).
Step 2
Brush bottom and sides of soufflé dish with all of the butter (it will look like a lot), going all the way to the top edges. Sprinkle bottom and sides with ¼ cup (56 g) superfine sugar and tilt dish to coat in an even layer; tap out excess. Chill until ready to use.
Step 3
Place a rack in middle of oven and preheat to 475°. Place 3.5 oz. bittersweet chocolate (70% cacao), chopped, in a medium heatproof bowl. Heat ½ cup heavy cream in a small saucepan over medium, stirring occasionally, until steaming, about 2 minutes. Pour cream over chocolate and let sit, stirring occasionally with a rubber spatula, until chocolate is melted and mixture is smooth and slightly cooled, about 5 minutes. Add 3 large egg yolks, room temperature, one at a time, whisking until smooth between additions.
Step 4
Stir ½ cup (50 g) Dutch-process cocoa powder, 1 Tbsp. dark rum (if using), ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt, and ½ cup hot water in a measuring glass until combined. Pour cocoa mixture into chocolate mixture and whisk to combine.
Step 5
Using an electric mixer on medium-high speed, beat 3 large egg whites, room temperature, ¼ tsp. cream of tartar, and ⅓ cup (75 g) superfine sugar in a large bowl until glossy and stiff peaks form, 5–7 minutes.
Step 6
Gently fold chocolate mixture into egg whites until just combined. Transfer to prepared soufflé dish and evenly sprinkle about 1 Tbsp. superfine sugar over.
Step 7
Bake soufflé until puffed and top is slightly darkened and just set with a slight wobble in the center when gently shaken, 25–30 minutes.
Step 8
To serve, dust soufflé with powdered sugar and spoon onto plates. Drizzle with heavy cream.
