In stock pot, melt butter over medium-high heat.
Add onion, celery, and bell pepper, and cook until softened, about 7 minutes.
Add flour, and cook for 1 minute more, stirring constantly.
Stir in chicken broth and purple sweet potato. Bring to a boil. Reduce heat, and simmer until sweet potato is tender, about 20 minutes.
Using immersion blender (or normal blender), blend until smooth.
Add sweet potato mixture to blender in batches. Remove center part of blender lid to allow steam to escape; secure lid on blender. Place a clean towel over opening in lid to avoid spills. Process until soup is smooth; return to pot.
Add tasso, cinnamon, nutmeg, and parsley. Bring soup to a simmer, and cook until thickened, about 20 minutes.
Stir in cream, and season to taste with salt and pepper.