Preheat oven to 375°F (190°C). Butter and sugar a 6-cup soufflé dish.
Melt chocolate and butter in a double boiler or microwave. Stir until smooth.
Whisk egg yolks and sugar until pale and thick. Add to the chocolate mixture.
In a separate bowl, beat egg whites with salt until stiff peaks form.
Gently fold one-third of the egg whites into the chocolate mixture to lighten it. Then, fold in the remaining egg whites until just combined.
Pour batter into the prepared soufflé dish. Bake for 25-30 minutes, or until puffed and set but still slightly soft in the center.
Dust with powdered sugar and serve immediately.
