Butter, room temperature, for baking dish
1/2 cup granulated sugar, plus more for baking dish
6 large eggs, room temperature, separated
6 ounces semisweet chocolate, melted
1/2 teaspoon salt
Confectioners’ sugar, for dusting
Preheat oven to 350 degrees, and set a kettle of water to boil. Butter a shallow 2-quart baking dish; coat with granulated sugar, tapping out excess.
In a large bowl, whisk egg yolks with granulated sugar until lightened in color. Whisk in melted chocolate; set aside.
In a large, clean bowl, using an electric mixer on high, beat egg whites with salt until soft peaks form. Whisk 1/3 of whites into chocolate mixture. Add remaining whites, and gently fold with a rubber spatula just until combined (do not overmix).
4. Transfer batter to prepared baking dish. Set dish in a roasting pan, and pour enough boiling water into pan to come about 1 inch up side of dish. Bake until puffed and just set (center of cake should barely move when jiggled), 25 to 35 minutes. Let cool 5 minutes; dust with confectioners’ sugar just before serving.