Brun smör chokladsoufflé för en folkmassa

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1. 1
   Cook 4 Tbsp. 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, stirring halfway through, until spreadable, about 20 minutes (butter should be opaque but still soft enough to brush).
2. 2
   Using a pastry brush, generously coat bottom and sides of soufflé dish with butter, making sure to go all the way to the edges. Sprinkle bottom and sides with some granulated sugar (about 2 Tbsp.) and tilt dish to coat evenly; tap out excess. Chill until ready to use.
3. 3
   Place a rack in middle of oven and preheat to 375°. Place 7 oz. (200 g) bittersweet chocolate (65–70% cacao), chopped, in a medium heatproof bowl. Heat 1 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 heatproof rubber spatula, until chocolate is melted and mixture is smooth and has cooled slightly, about 5 minutes. Add ½ cup (57 g) Dutch-process cocoa powder, 2 Tbsp. dark rum (if using), 1½ tsp. Diamond Crystal or ¾ tsp. Morton kosher salt, and ½ cup hot water; whisk to combine. Add 6 large egg yolks, room temperature, and whisk until smooth (don’t overwhisk; the objective is to incorporate, not aerate).
4. 4
   Using an electric mixer on medium-high speed, beat 6 large egg whites, room temperature, ½ tsp. cream of tartar, and 1 cup (200 g) granulated sugar in a large bowl until glossy and medium peaks form, 8–10 minutes.
5. 5
   Add one fourth of egg whites to chocolate mixture and mix with a spatula to incorporate completely (this will lighten the batter). Add remaining egg whites in 3 more batches, gently folding after each addition until only a few streaks of egg whites remain. Scrape batter into prepared soufflé dish.
6. 6
   Bake soufflé until puffed and top is slightly darkened and just set (it should wobble slightly in the center when gently shaken), 35–40 minutes.
7. 7
   To serve, dust soufflé with powdered sugar and spoon onto plates. Drizzle with heavy cream.