In a soup pot add butter and olive oil. Cook over medium heat until butter has melted. Add in onions. Cook 3 - 5 minutes or until onions are softened. Stir in curry paste, cook 1 minute. Pour in vegetable broth. Stir in pumpkin and soy sauce. Reduce heat to low and simmer for 15 minutes.
Slowly pour in coconut milk. Allow to cook for another 15 - 20 minutes. Season to taste with salt and pepper. Use an immersion blender to make the creamiest soup or carefully transfer hot soup to a blender.
Ladle into warm soup bowls. Top with cilantro if desired. Serve.
Notes
Be sure to use canned pumpkin puree not pumpkin pie mix. Blending the soup is going to give you the richest texture but it is optional. If you use a traditional blender, please carefully transfer the hot soup. You'll probably need to do it in batches as to not overload the blender.