Preheat the oven to 350 degrees F. Butter two 2-cup or one 4-cup souffle 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 the remaining tablespoon sugar, until they are very stiff and glossy. Stir a good spoonful of whites thoroughly into the egg yolk mixture to lighten it; then fold in the remaining whites, using a rubber spatula. Transfer to prepared souffle dish(es); at this point you can cover and refrigerate until you are ready to bake.
Bake until the center is nearly set, 20 minutes for individual souffles and 25 to 35 minutes for a single large souffle. Serve immediately.