Generously butter four 1- to 1 1/4-cup soufflé dishes or one 1 1/2-quart soufflé dish. If using small dishes, set them slightly apart in a 10- by 15-inch baking pan.
In a 2- to 3-quart pan over medium heat, melt 3 tablespoons butter. Add flour; stir until mixture is smooth and bubbling. Stir in milk; continue stirring until sauce boils and thickens, about 2 minutes. Remove from heat.
Add chocolate and stir until smooth. Add egg yolks and rum and stir until mixture is blended and smooth.
In a bowl, with a mixer on high speed, beat egg whites (use whisk attachment if available) with cream of tartar until foamy. Gradually add sugar and continue beating until short, stiff, moist peaks form. With a flexible spatula, fold a third of the chocolate sauce into whites until well blended. Add remaining sauce and gently fold in just until blended.
Scrape batter into prepared soufflé dishes; if higher than 3/4 full, use foil collar (see "Crowning Glory" below).
Bake in a 375° regular or convection oven to desired doneness: For a soft, creamy center, bake until edges feel set and dry but center 1 to 1 1/2 inches of small soufflés or 2 to 3 inches of large one still appear soft and jiggle slightly when dishes are very gently shaken, 12 to 15 minutes for small soufflés, 15 to 20 minutes for large one. For a fully set center, bake until surface appears set and fairly dry, 2 to 3 minutes longer for small soufflés, 5 to 6 minutes longer for large one.
Serve at once, scooping portions from single soufflé with a large spoon. Offer whipped cream to add to taste.
Soufflés look most impressive when they rise dramatically over the rim of the dish. To create a beautiful crown on your soufflé, fill the dish about 3/4 full. If it's less full, the soufflé may not rise over the rim. If it's more full, the soufflé may spill over unless you wrap the dish with a foil collar. Here's how to make one.
Cut a 15-inch-wide sheet of foil 4 inches longer than circumference of dish; fold lengthwise in thirds. Coat one side of the foil strip generously with melted butter, using a pastry brush. Wrap the foil around outside of dish so that at least 2 inches of foil extend above the rim. Fold the ends of the buttered foil strip over several times until snug against dish.