Use a needle or sharp knife point to pierce the skin of the fat ducks all over, taking care to not pierce the meat itself; go in at an angle. This helps the fat render out of the bird. Salt the ducks well and preheat the oven to 325 degrees Fahrenheit.
Put the ducks in an ovenproof pan. I rest them on celery leaves to prop them above the level of the pan; this helps them crisp better. If you want, surround the duck with some root vegetables. Roast for 90 minutes.
Take the pan out and increase the heat to 425 degrees. When it hits this temperature, put the birds back in the oven and roast until the skin is crispy, about 15 to 20 minutes.
Meanwhile, boil the vinegar and sugar in a small pot until it turns brown. Pour in the stock little by little, stirring all the while. Set aside.
Shave the peel off the oranges, grating some fine and keeping the peel of 2 oranges in large pieces. Juice 2 oranges. Cut segments from the other 2 oranges. Here is a tutorial on how to do that.
When the ducks are ready, remove them from the oven and let them rest on a cutting board.
Finish the sauce. Bring it to a simmer, then add about 1/2 cup of orange juice and the large bits of peel. Simmer 5 minutes. Whisk together a little of the sauce with the starch, and, when it's mixed well, stir it into the saucepot to thicken. Add the Grand Marnier and enough salt and orange bitters to taste. Swirl in the butter one tablespoon at a time.
To serve, carve the duck and arrange on plates. Give everyone some orange supremes and pour over the sauce. Garnish with the grated zest, and serve with good bread, mashed potatoes or celery root, or polenta.