Bring first 5 ingredients to a boil in a heavy saucepan over medium heat (about 20 minutes), whisking constantly; boil, whisking constantly, 1 minute or until thickened. Remove from heat; stir in butter and vanilla. Place heavy-duty plastic wrap directly on surface of custard (to keep "skin" from forming); chill 2 hours.
For Vanilla Custard Sauce, decrease cornstarch to 1 tablespoon; prepare as directed. Serve with strawberries or cake.