About 1 tablespoon butter for dish
1/3 cup sugar, plus some for dish
3 eggs, separated
2 ounces good quality bittersweet chocolate, melted
1/4 teaspoon cream of tartar
Preheat oven to 350 degrees. Butter two 2-cup or one 4-cup soufflé or other deep baking dish(es). Sprinkle each with sugar, invert it and tap to remove excess sugar.
Beat egg yolks with all but 1 tablespoon sugar until very light and very thick; mixture will fall in a ribbon from beaters when it is ready. Mix in the melted chocolate until well combined; set aside.
Wash beaters well, then beat egg whites with salt and cream of tartar until whites hold soft peaks; continue to beat, gradually adding remaining tablespoon sugar, until they are very stiff and glossy. Stir a good spoonful of whites thoroughly into egg yolk mixture to lighten it; then fold in remaining whites, using a rubber spatula. Transfer to prepared soufflé dish(es); at this point you can cover and refrigerate until you are ready to bake.
Bake until center is nearly set, 20 minutes for individual soufflés and 25 to 35 minutes for a single large soufflé. Serve immediately.