Pour 6 1/2 ounces of the orange juice into an ice cube tray and freeze until solid, approximately 2 hours. Store the remaining 2 ounces of juice and the zest in an airtight container in the refrigerator until ready to use.
In the carafe of a blender combine the frozen juice cubes, reserved orange juice and zest, milk, powdered sugar and vanilla, and blend until smooth and slushy, 30 to 45 seconds. Serve immediately.
Recipe courtesy of Alton Brown, 2009