This is a very simple, rich dish with a terrific crispy duck skin and nutty butter garnish on top. Duck cracklings are made from the leftover skin after dressing a duck. Ask your butcher if he will sell you just the skin, or buy a duck or duck breasts and save the meat for another use. Alternatievly, you can substitute crisped pancetta for the cracklings.
For the soup:
4 medium sweet potatoes (about 2 lbs. total)
2 Tbs. unsalted butter
2 Tbs. olive oil
1 small yellow onion, finely chopped (3/4 cup)
1 qt. lower-salt chicken broth
3/4 cup heavy cream
1/4 cup buttermilk
For the duck cracklings:
1 lb. duck skin (from 1 medium duck or 4 duck breasts)
1 bay leaf
For the sorghum brown butter:
2 oz. (4 Tbs.) unsalted butter, cut into cubes
2 Tbs. fresh lemon juice
2 Tbs. sorghum syrup
1/2 Tbs. finely chopped fresh flat-leaf parsley
Make the soup:
Position a rack in the center of the oven and heat the oven to 350ºF. Place the sweet potatoes on a baking sheet, and bake until tender, 40 to 50 minutes. When cool enough to handle, peel and cut the sweet potatoes into 3/4-inch pieces. Set aside.
In a large (4-quart) saucepan, heat the butter and oil over medium low heat. Add the onion and cook, stirring occasionally, until softened, 10 to 15 minutes.
Add the reserved potatoes, chicken broth, and 1 tsp. salt. Bring to a boil over medium-high heat, and then reduce the heat to medium-low or low and simmer for 15 minutes. Let cool briefly.
Working in batches, puree the soup in a blender until smooth. Strain through a fine mesh sieve into a clean sauce pan. Add the heavy cream and buttermilk, and season to taste with salt.
Make the duck cracklings:
Slice the duck skin into 1/2-inch strips and place in a 10-inch heavy-duty straight-sided sauté pan. Add 1/4 cup water, the bay leaf, and 1 tsp. salt. Bring to a boil over medium high heat, reduce the heat to maintain a simmer, and cook until the water evaporates and the skin is golden brown, 10 to 15 minutes. With a slotted spoon, transfer the skin to paper towels to drain. Season to taste with salt and let cool. Chop coarsely.
Make the sorghum butter:
In a 1-quart heavy-duty saucepan, heat the butter over medium heat until the milk solids turn golden brown, 5 to 6 minutes. Remove from the heat, and immediately add the lemon juice, sorghum, parsley, and 1/4 tsp. salt.
To serve, reheat the soup, if necessary. Ladle the soup into 6 bowls. Spoon about 1 Tbs. sorghum brown butter over the top and sprinkle about 2 Tbs. of duck cracklings over each.
Make Ahead Tips
The soup can be made up to 3 days ahead and refrigerated. Garnish with the sorghum butter and cracklings just before serving.