Slice vanilla beans in half. In a saucepan bring milk, cream, cinnamon sticks, vanilla beans, and zests to a simmer for a few minutes. Discard cinnamon and vanilla beans.
In a bowl whisk the egg yolks together with the cornstarch and 2 cups sugar until incorporated and creamy. Slowly add this mixture into the saucepan of warm milk, mixing continuously.
Slowly heat the mixture over low heat, stirring the entire time, until it begins to thicken. Make sure that it does not boil. Pour into 12 shallow serving dishes, cool to room temperature, and refrigerate for several hours.
To serve sprinkle cold custards with the remaining sugar and caramelize with a torch or under a hot broiler.
Recipe courtesy of Michael Chiarello, 2007