Preheat the oven to 425°F (218°C). Use a very sharp knife to slice the butternut squash in half, then scoop out the seeds using a spoon.
Bake the squash. Place each squash half, cut side up, on a baking sheet and rub each with a bit of the olive oil or avocado oil and season well with salt and pepper. Bake for 45-55 minutes, or until the flesh is completely fork tender and lightly browned.
Meanwhile, make the crispy breadcrumb topping. In a small bowl, combine the panko bread crumbs, nutritional yeast, salt, paprika, and melted vegan butter until well combined.
Meanwhile, cook the elbow macaroni. Bring a large saucepan of water to a boil and salt generously. Cook the pasta according to the box directions until al dente and drain in a colander. Toss with a little bit of olive oil to prevent sticking.
Once the squash is done roasting and cool enough to handle, scoop out the flesh and discard the skin. Measure out 1 1/2 cups (310-330g) of the flesh. Reserve the rest for another use, such as a curry, soup, or side dish.
Reduce the oven temperature to 400°F (205°C). Lightly grease a 3-quart/3-liter baking dish with oil or melted vegan butter.
Make the cheese sauce. In a high-powered blender, add the 1 1/2 cups (310-330g) of the squash flesh. Add in the soaked and drained cashews, nutritional yeast, mustard powder, garlic powder, onion powder, paprika, miso paste, tapioca flour or cornstarch, lemon juice, lite coconut milk, 1 1/2 teaspoons kosher salt, and black pepper to taste. Blend until the sauce is completely smooth and creamy, about 1-2 minutes, scraping down the sides as you go. Taste for seasonings and adjust accordingly, adding more salt as needed, more miso paste for more umami, more nutritional yeast for more cheesiness, or more lemon juice for more acidity.
Return the cooked and drained pasta back to its saucepan and pour in the cheese sauce. Toss until well combined. Transfer the mac and cheese to the prepared baking dish. Sprinkle the crispy breadcrumb topping evenly over the top.
Transfer to the oven and bake for 18-20 minutes, until the topping is crispy and brown. Allow to cool for 10-20 minutes to help set up, then slice into it and serve.