In a large pot, combine the butternut squash, cauliflower, and vegetable broth. Bring to a boil, then reduce heat and simmer for 20-25 minutes, or until the vegetables are tender.
Carefully transfer the soup to a blender (or use an immersion blender) and blend until smooth.
Return the soup to the pot and stir in the butter, curry powder, salt, and pepper.
Heat through and serve. Garnish with a dollop of sour cream, if desired.