Fill a large pot with water, add salt, and bring it to a boil. Cook the pasta shells for about 7-8 minutes, then drain, reserving 1 cup of pasta water.

In a large skillet, melt the butter over medium heat. Add the evaporated milk and bring to a simmer, stirring frequently.

Whisk in the pumpkin puree, mustard, and nutmeg, mixing until smooth and slightly thickened.

Stir in the cheddar and parmesan cheese gradually, continuously until fully melted and the sauce is creamy.

Combine the drained pasta shells into the cheese sauce, folding gently, and add reserved pasta water if needed to adjust texture.

Season with salt and pepper, serve hot, and garnish with chopped chives.