Remove the duck legs from the refrigerator and dust off any excess salt.
Preheat the oven to 300 degrees F.
Melt the duck fat in a large oven-proof casserole dish. Submerge the drumsticks completely in the fat and cover with aluminum foil or a lid. Bake the drumsticks until tender, 3 to 4 hours. With just a little pressure from your finger the meat should pull away from the bone. Remove the pan from the oven and set aside at room temperature for later use.
In a large saute pan, cook the onions in the butter. Deglaze with the brandy and flambe until the alcohol has burned off. Add the white wine and reduce until evaporated. Add the duck stock and maple syrup, and cook for 10 minutes over medium heat. Strain through a fine mesh chinois into a bowl, to ensure that the sauce is smooth. Incorporate the mustard into the sauce and set aside for later use.
Heat the duck fat until a deep-frying thermometer inserted in the fat reaches 375 degrees F. Mix the cornstarch and flour. While the oil is heating up, take the drumsticks and dip them into the egg whites, and then dip them into the cornstarch mixture.
Fry the battered drumsticks in duck fat until crispy and golden, about 6 minutes. Dip the fried drumsticks in the reserved sauce and then serve immediately.