In large saucepan, combine squash, broth, water and pepper. Cook over medium heat until squash is tender, about 20 minutes.
Blend until smooth (do this by very carefully transferring batches to a blender, using an submersion blender, or, what I do is use a potato masher - I don't mind the slightly lumpy texture at all, and actually prefer it).
Bring mixture just to boiling; reduce heat and simmer uncovered for about 5 minutes. Add butter; stir until melted.