SERVINGS: 4 to 6 servings
Preheat the oven to 425 F.
Add butternut squash to a sheet pan. Drizzle with olive oil, toss to coat, and season with salt. Roast until tender and caramelized in spots, about 20 minutes.
Transfer the cooked squash to a large liquid measuring cup or medium bowl and add 1/2 cup reserved pasta water. Blend with an immersion blender until pureed. Alternatively, you can do this in a blender or food processor.
Heat a large skillet over medium heat. Add the butter and let melt. Add the black pepper and toast until fragrant, 45 seconds.
Add the squash puree, spaghetti, Parmesan, 3/4 cup reserved pasta water, and salt to taste. Toss to combine and melt the cheese. If needed, add more pasta water.
Top with more Parmesan and black pepper before serving.