Bring a large pot of salted water to a rolling boil over high heat. Add the pasta shells and cook according to package directions until just shy of al dente, about 8-10 minutes. Drain the pasta, reserving 1 cup of starchy cooking water.

In a large skillet, melt 4 tablespoons of unsalted butter over medium heat. Once the butter is melted and bubbling, pour in 1 can of evaporated milk and stir well. Bring to a gentle simmer.

Reduce heat to low and whisk in 1 cup of pumpkin puree, 1 tablespoon of yellow mustard, and a pinch of nutmeg. Stir continuously until well combined, then gradually add 1 cup of shredded cheddar cheese and 1/2 cup of grated Parmesan. Stir until the cheese has melted and the sauce is smooth.

Add the drained pasta shells directly into the cheese sauce, gently folding until fully coated. Stir in reserved pasta cooking water as needed for desired consistency.

Season with salt and pepper to taste. Serve immediately, garnished with chopped chives.