In a large nonstick pot, melt butter and sauté onion until tender, about 4 minutes. Add sweet potato and chicken broth and bring to a boil. Reduce heat and simmer, covered, 15 minutes, or until potatoes are tender.
Pour mixture into a food processor and blend until smooth; return to pot. Add thyme, cayenne and milk and cook over low heat just until heated through. Addsalt and pepper. Makes 4 cups.