Melt the butter in a medium saute pan over medium-low heat. Add the onions, garlic and curry powder and cook, stirring occasionally, until the onions are translucent, about 10 minutes.
Combine the onion mixture, pumpkin puree and chicken stock in a blender until smooth.
Transfer the soup to a large pot, whisk in the coconut milk and cook over medium heat, stirring occasionally, until it warmed through, about 10 minutes. Taste the soup and season with the salt and pepper.
Serve the soup in bowls. Garnish with the sour cream and chopped pumpkin seeds. Serve with lime wedges, if desired.
This soup can be made vegetarian by swapping vegetable broth for the chicken stock.