Cook the pasta. Bring a large pot of water to a boil. Cook the macaroni according to package directions until tender. Reserve about 1/2 cup of the pasta water, then drain.
Blend the sauce. While the pasta cooks, add the butternut squash and the rest of the ingredients into a high-speed blender. Blend until completely smooth and creamy.
Cook the sauce. Pour the blended mixture into the same pot used for the pasta. Cook over medium-low heat for 3–5 minutes, stirring frequently, until the sauce is heated through and thickened. Do NOT bring to a boil.
Combine. Add the cooked macaroni and stir until evenly coated. If the sauce is too thick, add reserved pasta water or milk, a tablespoon at a time, until it reaches your desired consistency.
Notes
Butternut squash: Frozen butternut squash is such a convenient time-saver, but you can absolutely use fresh! Simply peel, remove the seeds, and cut into small cubes. Steam or boil for about 10–15 minutes, or until fork-tender. You can also roast at 400°F for 25–30 minutes until soft. Let cool slightly before adding to the food processor with the remaining sauce ingredients.
Cornstarch - You can leave it out! It helps thicken and stabilize the sauce, but it's not essential. I've tested this recipe without it, and while the sauce is still creamy, it does turn out a bit thinner and is more likely to separate when reheated.
Butternut squash: Frozen is a convenient shortcut! Make sure it's cooked until tender before blending. Fresh butternut squash works just as well.
Cottage cheese: Whole-milk cottage cheese gives the sauce a richer, creamier texture. So don't go for the low fat!