1. Pour 1/2 cup milk into a medium saucepan over medium heat. Scrape seeds from vanilla bean; add seeds and bean to milk. Cook 6 minutes (do not boil); discard bean. Combine 2 tablespoons granulated sugar and egg in a bowl. Gradually add hot milk mixture to bowl, stirring constantly with a whisk. Return the mixture to pan. Cook over medium heat for 4 minutes or until mixture coats the back of a spoon, stirring constantly. Immediately pour into a bowl. Cover and chill.
2. Place a baking sheet in oven. Preheat oven to 425°.
3. Lightly coat 6 (8-ounce) ramekins with cooking spray. Sprinkle evenly with 2 1/2 tablespoons granulated sugar, tilting and turning dishes to coat sides.
4. Combine brown sugar, flour, and salt. Place butter in a medium heavy saucepan over medium heat; cook for 3 minutes or until butter browns slightly. Stir in flour mixture and 1 1/4 cups milk; bring to a boil. Cook for 2 minutes or until slightly thick, stirring constantly, and remove from heat. Let stand 5 minutes. Stir in vanilla extract and egg yolk.
5. Combine egg whites and cream of tartar in a large bowl; let stand at room temperature 15 minutes. Beat with a mixer at high speed until medium peaks form. Gently stir one-fourth of egg whites into milk mixture; gently fold in remaining egg whites. Gently spoon mixture into prepared dishes. Sharply tap dishes 2 or 3 times on counter. Place dishes on preheated baking sheet, and return baking sheet to 425° oven. Immediately reduce oven temperature to 350°; bake soufflés at 350° for 30 minutes or until puffy and golden. Serve immediately with sauce.