Melt butter in a medium-sized saucepan. Add chopped onion and, stirring frequently, cook until soft but not brown.
Add curry powder and cook 1 to 2 minutes longer. Place curried onion in a food processor or blender. Add pumpkin and salt and process until smooth.
Add half-and-half and process again until smooth. Pour pumpkin mixture back into saucepan and stir in vegetable broth. Heat soup slowly over low heat, stirring occasionally.
Meanwhile, stir cinnamon and minced parsley into the sour cream. Serve soup steaming hot with a dollop of seasoned sour cream in each bowl.