Place 1/2 cup sugar in a small heavy saucepan over medium heat, and cook until sugar is golden (about 5 minutes). Immediately pour sugar syrup into a 1 1/2-quart soufflé dish coated with cooking spray, tipping quickly until caramelized sugar coats bottom of dish; set aside.
Combine milk and 1/2 cup sugar in a large heavy saucepan; stir well. Scrape seeds from vanilla bean; add seeds and bean to milk mixture. Cook over medium-high heat to 180° or until tiny bubbles form around edge (do not boil). Remove from heat (add extract at this point if using in place of vanilla bean); cover and let stand 10 minutes. Discard bean.
Preheat oven to 375°.
Place milk mixture over medium heat. Gradually add flour and salt, stirring constantly with a whisk. Cook 12 minutes or until thick and bubbly, stirring constantly. Gradually add hot milk mixture to eggs, stirring constantly with a whisk; stir in nutmeg. Spoon mixture into prepared dish.
Bake at 375° for 45 minutes or until puffy and almost set. (Cake will be slightly "jiggly" in center.) Cool in dish 5 minutes. (Cake will deflate slightly upon standing.) Loosen cake from sides of dish using a narrow metal spatula. Place a plate upside down on top of cake; invert on to plate. Serve warm or chilled.