Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius).
Cut the butternut squash in half lengthwise and scrape out the seeds with a spoon.
Place the squash halves cut-side down on a parchment-lined baking sheet.
Roast for 45 minutes or until the flesh is very tender when pierced with a fork.
Allow the squash to cool slightly, then scoop the flesh out of the skin and into a food processor.
Add butter, salt, pepper, and nutmeg to the processor.
Pulse until the mixture is completely smooth and velvety.
Serve immediately or reheat in a saucepan over low heat.