Pour milk into a heavy saucepan. Scrape seeds from vanilla bean; add seeds and bean to milk. Cook over low heat 20 minutes; discard vanilla bean. Combine 1/4 cup sugar and cornstarch; add to milk mixture, stirring well.
Gradually stir about one-fourth of hot milk mixture into egg yolk; add to remaining hot milk mixture, stirring constantly. Bring to a boil over medium-high heat, stirring constantly. Cook 1 minute, stirring constantly (mixture will not be thick). Pour into a bowl; cool slightly, and cover surface with plastic wrap.
Chill thoroughly (mixture will thicken slightly). Coat a 1-quart metal bowl with cooking spray; sprinkle with 1 teaspoon sugar. Set aside. Beat egg whites (at room temperature) at high speed of an electric mixer until foamy. Gradually add 1/2 cup sugar, 1 tablespoon at a time, beating until stiff peaks form.
Add vanilla extract, and beat 10 seconds. Spoon mixture into prepared bowl; gently smooth surface of mixture with a spatula. Place bowl in an 8-inch square baking pan; add hot water to pan to a depth of 1 inch. Bake at 325 deg for 20 minutes or until slightly puffed and set.
Invert island into a shallow dish. Pour sauce around island, and sprinkle with grated chocolate.