Bring a large pot of salted water to a boil. Add the diced butternut squash and cashews and cook until tender, 10–12 minutes.
Drain the butternut squash and nuts in a colander, discarding the water.
Transfer the drained squash and cashews to a blender along with the tapioca starch, plant-based milk, vinegar, vegan butter, nutritional yeast, smoked paprika, mustard, onion, and salt. Blend until completely smooth, forming the cheese sauce.
Bring 12 cups of water to a boil. Once boiling, add the salt and the macaroni. Cook until al dente according to package directions.
Drain well and return the pasta to the pot. Pour the butternut squash cheese sauce over the pasta and stir until evenly coated.
For the optional topping, preheat the oven to 375°F (190°C). Toss the thinly sliced squash with poultry seasoning, tamari, and olive oil. Spread on a baking sheet and roast until tender and lightly browned, 20–25 minutes.
Spread the mac 'n cheese into a casserole dish. Top with the roasted squash and breadcrumbs. Bake until the breadcrumbs are golden and crisp, 12-14 minutes.
Remove from the oven, sprinkle with chives, and serve hot.