Bring a large pot of generously salted water to a boil.
Add the pasta shells or macaroni and cook until just shy of al dente according to the package directions.
Before draining, reserve about 1 cup of the starchy pasta cooking water. Drain the pasta and set aside.
While the pasta cooks, 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 cheddar and Parmesan, stirring continuously until the cheese has melted and the sauce is creamy.
Season the pumpkin cheese sauce lightly with salt.
Add the drained pasta directly to the skillet. Stir gently until every piece is thoroughly coated in the sauce.
If the Easy Pumpkin Mac and Cheese Recipe seems too thick, add reserved pasta water a few tablespoons at a time until the sauce reaches your preferred consistency.
Taste and season with additional salt and freshly ground black pepper.
Serve immediately, topped with finely chopped fresh chives if desired.