In a large saucepan, heat butter over medium-high heat; sauté onion and ginger until tender. Add the stock, pumpkin and seasonings and then whisk until blended. Bring the soup to a boil. Reduce the heat; simmer, covered, until the flavors are blended, which is about 15 minutes.
Let the soup cool slightly and puree soup in batches in a blender; return to pan. Stir in the coconut milk, heat through, and serve when desired.
