This soufflé doesn't rise over the edge of the dish, but it's rich and delicious, and its creamy sauce balances the chocolate.
To prepare sauce, heat 1 1/4 cups milk and 1/2 vanilla bean in a small, heavy saucepan over medium heat to 180° or until tiny bubbles form around edge (do not boil), stirring frequently. Remove from heat; let stand 30 minutes. Scrape seeds from vanilla bean, and add seeds to milk. Discard bean.
Combine 3 tablespoons sugar, 2 egg yolks, and dash of salt in a medium bowl, stirring with a whisk. Gradually add milk mixture to the egg mixture, stirring constantly with a whisk. Return milk mixture to pan. Heat milk mixture over medium heat to 180° or until tiny bubbles form around edge (do not boil), stirring constantly with a whisk. Reduce heat; cook 2 minutes or until slightly thick and mixture coats the back of a spoon, stirring constantly. Cover and chill.
To prepare the soufflé, preheat the oven to 400°.
Lightly coat a 2-quart soufflé dish with cooking spray. Coat with 2 tablespoons sugar; set aside.
Heat 1 cup milk and 1/2 vanilla bean over medium heat in a small, heavy saucepan to 180° or until tiny bubbles form around edge (do not boil), stirring frequently. Remove from heat; let stand 30 minutes. Scrape seeds from vanilla bean. Stir seeds into milk; discard bean. Add 1/2 cup sugar, cocoa, cornstarch, and 1/8 teaspoon salt to milk mixture, stirring with a whisk. Bring to a boil over medium heat, stirring frequently. Cook 1 minute or until thick, stirring constantly. Remove from heat, and stir in chocolate until melted. Let cool 5 minutes. Place 2 egg yolks in a large bowl; stir with a whisk. Gradually add chocolate mixture to egg yolks, stirring constantly with a whisk.
Place egg whites and cream of tartar in a large bowl; beat with a mixer at high speed until soft peaks form. Gradually add 2 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently stir one-fourth of egg white mixture into chocolate mixture; gently fold in remaining egg white mixture. Spoon into prepared dish. Bake at 400° for 10 minutes. Reduce oven temperature to 375° (do not remove soufflé from oven); bake an additional 25 minutes or until puffy and set. Serve immediately, and top each serving with two tablespoons sauce.