Preheat oven to 325°.
Combine 1/3 cup sugar and water in a small heavy saucepan over medium-high heat; cook 5 minutes or until golden, stirring occasionally. Immediately pour into 6 (6-ounce) custard cups coated with cooking spray, tipping quickly until sugar coats bottoms of cups; set aside.
Combine 1/2 cup sugar, eggs, and egg whites in a large bowl; stir well with a whisk. Add milks; stir until smooth. Scrape seeds from vanilla bean; stir seeds into milk mixture, reserving bean for another use. Divide mixture evenly among prepared cups. Place cups in a 13 x 9-inch baking dish; add hot water to a depth of 1 inch. Bake at 325° for 1 hour or until knife inserted in center comes out clean. Remove cups from pan; let cool completely on a wire rack. Cover and chill at least 3 hours.
Loosen edges of custards with a knife or rubber spatula. Place a dessert plate, upside down, on top of each cup; invert custards onto plates. Drizzle any remaining syrup over custards. Garnish with mint sprigs, if desired.