Melt butter in a large saucepan or soup pot over medium heat, and cook and stir the onion until translucent, about 5 minutes.

Pour in the chicken broth, and stir in the butternut squash cubes, orange juice, and orange zest. Bring to a boil, reduce heat, and simmer until the squash cubes are tender, 15 to 20 minutes.

Pour the soup into a blender, filling the pitcher no more than halfway full. Hold down the lid of the blender with a folded kitchen towel, and carefully start the blender, using a few quick pulses to get the soup moving before leaving it on to puree.

Puree in batches until smooth and pour back into the pot. Alternately, you can use a stick blender and puree the soup right in the cooking pot.