Preheat oven to 350F. Lightly butter a 9x5 loaf pan and line it with parchment paper with long ends so you can lift the bread out later for glazing and slicing.
Wash and dry the orange. Remove the peel with a vegetable peeler. A serrated peeler works best for this. You want to remove just the orange part of the peel, with little of the bitter white pith underneath.
Put the sugar and orange peelings in a food processor and process until the peels are completely incorporated into the sugar. You may want to pulse as well as process to get all the chunks of peel broken down. The sugar will be pale orange, and incredibly fragrant with a moist texture similar to brown sugar. Note: do not make your orange sugar ahead of time, use right away.
Put the orange sugar and butter into a stand mixer and cream until light and fluffy ~ a full 5 minutes.
Beat in each of the eggs, one by one, making sure each egg is fully incorporated before adding the next one. Scrape down the sides of the bowl and beat in the vanilla.
Whisk together the flour, cornstarch, baking powder, salt and cardamom and add to the mixer, alternately with the buttermilk, beginning and ending with dry ingredients. Finish mixing by hand, to make sure everything is thoroughly mixed, but don't over beat.
Turn the batter into the prepared pan, spread out evenly, and bake on the center rack for 55-60 minutes, or until fully risen and a toothpick inserted near the center comes out without wet batter clinging to it (moist crumbs are fine.) Ovens and pans vary so check your bread on the early side.
Let the bread cool in the pan for 10 minutes, then remove, using the parchment paper handles, and remove to a cooling rack.
Meanwhile whisk together the sugar with enough orange juice to make a thick glaze. Spread the glaze over the slightly warm or cooled bread. Wait until the glaze sets up before slicing.
Notes
The success of any cake or quick bread is in the details. Make sure your oven is accurate. Make sure you measure correctly. Use the fluff/scoop/level method for measuring flour: fluff up the flour first to loosen it, then scoop it with your cup measure, then lightly level it off with the flat side of a knife or your finger.Loaf pans are annoyingly different in size and shape. I use a classic 9x5 pan which has an 8 cup capacity. If your pan is at all different, your results and baking time may vary.Do not make orange sugar ahead of time, it should be used shortly after making.