Put sweet potatoes into a large pot. Pour chicken broth over the sweet potatoes; bring to a boil, stir orange liqueur into the boiling broth, and cook until the potatoes are tender, about 25 minutes.
Puree the sweet potatoes with the broth using an immersion blender until smooth.
Stir half-and-half, nutmeg, ginger, cinnamon, and cayenne pepper into the pureed sweet potatoes; season with salt and pepper.
Heat a skillet over medium-high heat. Toast pecans in the hot skillet until fragrant.
Ladle soup into bowls; garnish with toasted pecans.
