Pour 6 ½ fluid ounces of the orange juice Make sure you zest your oranges before collecting the juice. into an ice cube tray and freeze until solid, approximately 2 hours. Store the remaining 2 fluid ounces juice and the zest in an airtight container in the refrigerator until ready to use.
Combine the frozen juice cubes, reserved orange juice and zest, milk, confectioners' sugar, and vanilla in a blender and blend until smooth and slushy, 30 to 45 seconds. Serve immediately.
