Preheat the oven to 400 F. Place the squash on a rimmed baking sheet, drizzle over the melted butter, season with salt and pepper, and toss to combine. Spread out in a single layer on the baking sheet, allowing space in between the cubes when possible. Roast for about 35 to 40 minutes until tender and lightly browned on the bottom.
Heat the oil in a stockpot over medium heat. Add the shallots or onions and sauté for 10 minutes, until the shallots have softened and turned a light golden brown. Add the broth and bring to a simmer.
Add the cooked butternut squash to the simmering broth and, working in batches, puree the soup in a blender or food processor (alternately, you can run the mixture through a food mill, which offers a great kind of nubby texture, or puree right in the pot with an immersion blender). Return the pureed soup to the pot. Taste and adjust the seasoning.
Add the cream and heat just until warmed through. Ladle the soup into bowls, and dollop or swirl in a bit of the sour cream or crème fraiche, and sprinkle with some pumpkin seeds, crisped bacon, or chives, if desired.
Notes
For extra seasoning and depth of flavor, you could definitely add a pinch of nutmeg, or perhaps a bit of cumin and chili powder, or some fresh or dried sage.
If you don't have shallots, use chopped onions. All good.
If you have vegetarians joining you at the table, use vegetable broth and pass the bacon on the side, or just give the bowls a sprinkle of fresh parsley or chives instead.
Want to make this soup vegan or dairy-free? Add in some full-fat, unsweetened coconut milk instead of heavy cream. Make sure to use vegetable broth, and use the same amount of olive oil instead of butter for roasting the squash.
You could also use another kind of winter squash instead of the butternut. Some of the other winter squashes can be peeled and cut up similarly to butternut, but others don't peel and cube so easily (psst, learn about peeling and cutting up butternut squash). Take acorn squash, for example, with all of those little ridges and a skin that is super hard and thick. But what you can do with those types of squash is to cut them in half, scrape out the seeds and stringy gunk inside, and then roast them as is. Here is a nice little tutorial on how to do just that. Be really careful when cutting acorn squash; use a sharp knife and a sure hand.