Preheat the oven to 350°F and spray a 1.5-quart casserole dish with nonstick spray and set aside.
Bring a large pot of water to a boil with 1-2 tablespoons of salt and make the pasta according to the package directions until al dente. Drain the pasta in a colander when it's done.
While it's boiling, make the vegan cheese sauce. Add the vegan butter to a deep skillet over medium-high heat and swirl it around to melt. Add the minced garlic and cook for 1-2 minutes, until fragrant. Turn the heat down to medium and sprinkle the flour on top and whisk it well to create a thick paste, also known as a roux.
Slowly whisk in 3 cups of dairy-free milk, stirring frequently, until thickened and smooth. Turn the heat up slightly to medium-high and allow the mixture to bubble to help thicken it. Continue to whisk well, especially around the edges of the pan, to keep the cheese sauce smooth. Stir in 1/3 cup of the nutritional yeast, smoked paprika, and onion powder, and remove the sauce from the heat.
Add the remaining 1/3 cup nutritional yeast to a high-speed blender with the cashews, garlic powder, and salt. Pulse it a few times into small bits, like parmesan cheese. Remove 1/4 cup of the mixture to save for the topping.
Carefully pour the prepared cheese sauce into the blender with the remaining cashew mixture and blend until smooth, 1-2 minutes. It may take longer to blend smoothly if you don't have a high-speed blender (like a Vitamix).
Pour the cheese sauce back into the skillet over medium-low heat. Mix the arrowroot flour with 1/2 cup of milk in a separate small bowl to dissolve it. Pour it into the cheese sauce and turn the heat up to medium-high to bubble then turn it down and simmer for 1-2 minutes, stirring frequently, to thicken the sauce. The arrowroot gives it a stringy, cheese-like consistency. Whisk in more milk a few tablespoons at a time if it looks too thick. Remove from the heat.
Drain the pasta from the water, if you haven't already, and add it to the skillet with the cheese sauce and mix to coat it all. Pour it into the prepared baking dish and set aside.
To the same pot, or a new clean skillet, melt 2 tablespoons of vegan butter over medium-high heat. Add 1 cup of panko breadcrumbs, salt, and the reserved vegan parmesan cheese. Stir frequently for 3-4 minutes to toast until golden brown.
Sprinkle the toasted breadcrumbs over the vegan mac and cheese and bake in the preheated oven, uncovered, for 12-15 minutes.