Place the drippings and duck skin in a large skillet over medium-high heat. Add 3 cups of the eggplant and saute until eggplant starts to get soft, translucent and brown, about 5 minutes, stirring frequently. Add the onions and remaining 1 cup eggplant; cook until the onions start to brown, about 8 to 10 minutes, stirring occasionally. Add the finely chopped sweet potatoes; continue cooking and stirring for 4 minutes. Stir in the garlic and cook 3 minutes, stirring occasionally. Add the bay leaves, 3 teaspoons of Bayou Blast and stir well, scraping pan bottom as needed.
Next, stir 1 cup of the stock into the vegetables and cook 2 minutes, then add another 1 cup stock; cook 5 minutes, stirring occasionally. Stir in Â¼ cup of the sugar and cook 2 minutes, stirring occasionally. Add another 1 cup stock and cook 10 minutes, stirring occasionally. Add the remaining Â¼ cup sugar and 1 cup more stock; cook 10 minutes, stirring occasionally, then add another 1 cup stock and cook 10 minutes more, stirring occasionally. Reduce heat to low and simmer 13 minutes. Stir in another 1 cup stock and simmer 3 minutes more. Remove from heat and strain well, forcing as much liquid as possible through the strainer.
Place the strained gravy in a 2-quart saucepan. Add the diced sweet potatoes and 1 cup stock; bring to a boil over high heat, then reduce heat and simmer 3 minutes, skimming off any froth from the surface. Stir in the grand Marnier and continue simmering 7 minutes, stirring occasionally. Add the green onions, the remaining teaspoon of Bayou Blast and 1 cup more stock. Bring gravy to a boil and simmer until it reduces to about 3 cups, about 8 minutes, stirring occasionally.