1) Set the freezer to rapid-freeze at least 2 hours before freezing. To ensure that the oranges stand upright, cut a thin slice from the base of each. Cut a lid from each orange at the other end and set aside. Carefully cut down the inside of each orange and remove the pith (the white stuff) and flesh from each each. Do this over a bowl to catch all the juice.
2) Discard the pith from the scooped-out flesh, then chop the flesh to make a chunky puree. Place in a bowl with the juice. Stir in the buttermilk and the honey, and pour into a freezer-proof container. Freeze for 1 hour. Place the empty orange shells upside down on paper towels and let drain.
3) Remove the orange mixture from the freezer and stir well, breaking up any ice crystals. Return to the freezer for an additional 30 minutes, or until semi-frozen.
4) Stir again and use to fill the orange shells. Stand the filled oranges upright in a container. Freeze for an additional hour, or until frozen.
5) Before serving, transfer to the refrigerator for 30 minutes to soften slightly. Serve decorated with chocolate shavings, if using.