In a medium skillet, heat the butter over medium heat, stirring and swirling the pan occasionally. When the butter starts to foam and smell nutty, add the sage leaves. Stir in the panko until moistened. Cook for about 30 seconds so the panko crumbs can get a little toasty. Remove from the heat. Stir in the Parmesan cheese and season with salt and pepper. Set aside.

Bring a large pot of salted water to a boil. Add the pasta and cook it just until al dente, according to package instructions. Drain the pasta set aside.

While the pasta is cooking, melt the butter over medium heat in a large pot. Turn the heat to low and whisk in the flour, ground mustard, and garlic powder to create a paste. Whisk in milk until smooth. Turn the heat to medium and whisk until the sauce starts to thicken, about 2 minutes. Whisk in the pumpkin until smooth. Stir in the shredded cheese until the cheese is melted and the sauce is smooth.

Add the drained pasta and stir until well coated with the cheese sauce. Season with salt and black pepper, to taste. Add a pinch of nutmeg, if using.

Spoon the mac and cheese into bowls and top with the brown butter panko topping. Serve warm.