Add the butter to a medium sauté pan set over medium-low heat. Once the butter has melted, add the onions, garlic and curry powder and cook, stirring occasionally, until the onions are translucent. Transfer the mixture to a blender.
Add the pumpkin purée and chicken stock to the blender and blend until combined. Pour the soup into a large stock pot set over medium heat then whisk in the coconut milk. Cook the soup, stirring occasionally, until it is warmed throughout. Taste and season it with salt and pepper.
When ready to serve, divide the soup into bowls and garnish with sour cream and chopped pumpkin seeds.