Roast the Squash: Toss the cubed butternut squash with a little olive oil, salt, and pepper, and roast at 400°F (200°C) for 25–30 minutes, until fork-tender and lightly caramelized at the edges. This step can be done a day ahead if you want to speed up dinner later in the week.
Cook the Pasta: Bring a large pot of salted water to a boil and cook the pasta according to package directions. Before draining, scoop out and reserve about 1/2 cup of the starchy pasta water it's what helps the sauce come together smoothly later.
Brown the Butter and Sage: In a large skillet over medium heat, melt the butter and add the sage leaves. Cook for about 3 minutes, swirling occasionally, until the butter turns a deep golden color and smells nutty and toasted, and the sage leaves have crisped up. Watch closely in the last minute brown butter can go from perfect to burnt quickly.
Blend the Sauce: Add the roasted squash and heavy cream to a blender (or use an immersion blender directly in the skillet) and blend until completely smooth. Pour the blended squash sauce back into the skillet with the brown butter.
Bring It Together: Stir the Parmesan into the sauce until melted, then fold in the crumbled Italian sausage if using. Thin the sauce with the reserved pasta water, a splash at a time, until it reaches a silky, clingy consistency.
Toss and Serve: Add the cooked pasta directly to the skillet and toss until every piece is coated in sauce. Season with salt and pepper to taste, and serve warm with extra Parmesan and a few of the crisped sage leaves on top.