Cook the pasta according to the package directions, just until al dente. Transfer to a strainer, and run the pasta under cold water to stop it cooking. Set aside the cooled, cooked pasta.

In large a large skillet over medium heat, melt the butter. Whisk the flour into the butter, and let it cook out the flour taste. Whisk in the milk, one cup at a time, to form a roux. Bring the roux to a simmer and continue cooking, and whisking occasionally, until it's thickened a bit, about 5 minutes.

Whisk in the garlic powder, paprika, mustard, and salt and pepper, to taste. Stir in the pumpkin until the sauce is smooth. Add in the cheeses and stir until they've melted and the sauce is once again smooth. Turn off the heat, and transfer the pasta to the skillet. Gently toss to combine, until all the pasta is coated.

Transfer the macaroni and cheese to a casserole dish. Spread it out evenly and add some extra shredded cheddar cheese to the top, followed by the bread crumb topping. Dot the crumb topping with the additional butter.

Bake at 375 degrees for 20-30 minutes, or until the macaroni and cheese is bubbling and the crumb topping is lightly browned and crisp.