In a Dutch oven or large pot, combine the butternut squash, onions, carrots, and chicken stock. Heat over medium heat until the soup begins to simmer. Simmer the soup uncovered for 45 minutes until the vegetables are very soft.
Remove the pot from the heat. Carefully blend the soup with an immersion blender until smooth or puree in a blender in small batches, then transfer back to the pot.
Stir in the butter until melted then add the cream. Stir the soup until well blended. Season with salt and pepper.
Serve the soup hot with sour cream ot desired toppings.