Preheat oven to 375°F. Grease a 2-quart baking dish with 1 Tbs. of the butter. Cook macaroni according to package directions; drain and return to pot.
While macaroni cooks, melt the remaining 2 Tbs. butter in a large saucepan over medium heat. Add flour and whisk until combined. Whisk in milk, mustard, salt, and pepper. Cook and stir until thickened and bubbly. Add pumpkin and cheddar cheese. Cook and stir until creamy, 1 minute. Remove from heat.
Pour sauce over pasta and stir to coat. Transfer pasta to the prepared baking dish. In a small bowl, stir together breadcrumbs, Parmesan, and oil. Spread evenly over pasta. Bake for 15 to 20 minutes or until the topping is golden brown and pasta is heated through. Serve immediately.