In a 4 cup glass measuring cup combine the brown sugar, heavy cream and light corn syrup. Set aside in microwave until later.
To cut oranges into supremes: remove rind and pith. Over a bowl to catch the juices, cut in between skins to release supremes. Squeeze juice from skins over supremes and toss with a bit of cardamom to taste. Set supremes in 4 dessert bowls.
When ready to eat microwave brown sugar mixture, uncovered at full power for 2 minutes. Whisk to blend, and cook 1 minute more until a bit thickened. Remove from heat and stir in butter. Immediately pour over oranges and garnish with pistachios. Sauce will thicken as it cools. Leftovers can be kept in the fridge for a week. To soften, microwave 30 seconds to a minute, uncovered.