Saute chopped onion and grated ginger in 1 tablespoon of butter in large, heavy soup pot over medium-low heat for about ten minutes. Add cubed squash and cook for five minutes. Add chicken or vegetable stock and simmer until squash is tender, about 15 or 20 minutes. Add peanut butter and coconut milk and stir to blend. Puree in blender and return to soup pot to heat thoroughly. Garnish with fresh cilantro.