1/3 cup sugar plus additional for sprinkling
5 oz bittersweet chocolate (not unsweetened), chopped
3 large egg yolks at room temperature
6 large egg whites
Preheat oven to 375°F. Generously butter soufflé dish and sprinkle with sugar, knocking out excess.
Melt chocolate in a metal bowl set over a saucepan of barely simmering water, stirring occasionally until smooth. Remove bowl from heat and stir in yolks (mixture will stiffen).
Beat whites with a pinch of salt in a large bowl with an electric mixer at medium speed until they just hold soft peaks. Add 1/3 cup sugar, a little at a time, continuing to beat at medium speed, then beat at high speed until whites just hold stiff peaks. Stir about 1 cup whites into chocolate mixture to lighten, then add mixture to remaining whites, folding gently but thoroughly.
Spoon into soufflé dish and run the end of your thumb around inside edge of soufflé dish (this will help soufflé rise evenly). Bake in middle of oven until puffed and crusted on top but still jiggly in center, 24 to 26 minutes. Serve immediately.