Preheat oven to 375°F (190°C). Cut the butternut squash in half lengthwise and scoop out the seeds. Place the squash halves on a baking sheet and roast for 30-40 minutes, or until tender.
Once the squash is cool enough to handle, scrape the flesh from the skin and place it in a large bowl. Mash the squash with a fork or potato masher.
Add heavy cream, melted butter, Parmesan cheese, salt, and pepper to the mashed squash. Stir until well combined.
Transfer the squash mixture to a greased 9x13-inch baking dish. Sprinkle the panko bread crumbs evenly over the top, then drizzle with melted butter.
Bake in the preheated oven for 25-30 minutes, or until the casserole is golden brown and bubbly.
Let the casserole cool for a few minutes before serving.
