Bring a large pot of generously salted water to a boil. Add the pasta and cook until just shy of al dente according to the package directions.
Before draining, reserve about 1 cup of the pasta cooking water. Drain the pasta and set aside.
Meanwhile, melt the butter in a large skillet or saucepan over medium heat.
Pour in the evaporated milk and bring it to a gentle simmer, stirring occasionally.
Whisk in the pumpkin puree, yellow mustard, and nutmeg until the mixture is smooth and evenly combined.
Reduce the heat to low. Gradually add the grated sharp cheddar and Parmesan, stirring after each addition until the cheese has completely melted and the sauce is creamy.
Season the sauce with salt to taste.
Add the drained pasta and gently stir until every shell is coated in the pumpkin cheese sauce.
If your Easy Pumpkin Mac and Cheese is thicker than you'd like, gradually stir in some reserved pasta water until the sauce reaches your preferred consistency.
Taste and adjust with additional salt and freshly ground black pepper. Serve immediately with chopped fresh chives, if desired.