In large pot, melt butter over med heat & add onion. Cook 5 to 8 minutes until onion is softened. Add potato, broth, squash, salt and pepper, to taste. Bring to low boil & simmer for 30 minutes until squash & potato are very soft. Remove from heat & let cool slightly. Mash with potato masher or for smoother texture, transfer carefully to blender or food processor in small batches and puree until smooth. Adjust salt and pepper to taste. Serve in bowls with a dollop of sour cream and chopped fresh chives (both optional)