Preheat your oven to 375 degrees F.
Thoroughly coat your soufflé dish with melted butter.
Once coated with butter, dust bottoms and sides with sugar.
Place your chocolate in the top pan of a double boiler, over an inch of simmering water. Heat, stirring often, until the chocolate melts, without burning it. Remove from the water to let it cool.
Separate the egg whites and yolks.
Collect sugar, cream of tartar, and salt. Add salt and cream of tartar to the egg whites and beat at medium-high speed until soft-peaked. It's at soft peak when it can almost be formed into a peak.
(This is soft peak) Once at soft peak, slowly add the 1/4 cup of the sugar and beat until stiff, glossy peaks form.
In another bowl, whisk the egg yolks until thick and pale in color. Whisk in the remaining 1/2 cup of sugar and the vanilla.
With a rubber spatula, fold in the melted chocolate.
Fold one-fourth of the beaten egg whites into the chocolate mixture to lighten it.
Continue to fold in all egg whites until thoroughly combined.
Spoon or pour into the prepared soufflé dish until 5/6 full.
Place in the preheated oven and bake until puffed, set, and the center slightly jiggles when given a gentle shake. About 25-30 minutes.
Dust with powdered sugar and serve immediately with ice cream or whipped cream!