Preheat the oven to 375°F.
Lightly coat the interior of a 1 ½-quart soufflé mold with the butter. Place ¼ cup of the granulated sugar in the mold and begin turning the mold back and forth so that the entire interior is generously coated with sugar. Turn upside down to allow any excess sugar to fall out. Set aside.
Combine the egg whites with the remaining ½ cup granulated sugar in the bowl of a standing electric mixer, whisking to combine.
Fill a 1-quart saucepan halfway with water and place it over high heat. Bring to a boil. Place the bowl over the boiling water without allowing the bottom of the bowl to directly touch the water. Using a wire whisk, beat for about 3 minutes, or until quite warm.
Remove from the heat and place the bowl in the stand mixer fitted with the whip attachment. Beat on medium, just until frothy. Increase the speed to medium-high and beat until stiff but not dry peaks form.
Remove the bowl from the mixer and, using a rubber spatula, fold the melted chocolate into the meringue, taking care not to overmix, but also taking care that the egg whites are completely incorporated into the chocolate.
Carefully scrape the soufflé batter into the prepared mold. It should come to about 1 inch below the top. Place in the middle of the oven, making sure that it has room to rise without hitting a top rack. Bake for about 20 minutes, or until the soufflé has risen about one and a half times its height and the top is lightly colored.
Remove from the oven. Place the confectioners’ sugar in a fine-mesh sieve and, tapping on the sides of the sieve, dust the top of the soufflé with the sugar. Serve immediately with a bit of whipped cream, if desired.