Fill a medium pot with water and bring to a boil. Add in macaroni and cook until just done. Drain and pour back into pot.
In a separate small pot, add butter and flour. Cook on low heat, stirring constantly, until smooth slurry forms. Add heavy cream, cheese, pumpkin, nutmeg, mustard and pepper and continue to stir until smooth cheese sauce forms. I prefer to use fresh shredded cheese rather than buying pre-shredded, to keep the sauce from getting gritty. Once cheese sauce is uniform, stir in approximately 1/4 cup of milk to thin out sauce. If you feel your sauce is still too thick, you can add another 1-2 tbsp of milk.
Pour 2/3 of the cheese sauce into macaroni pot, reserving about 1/3 of the sauce, and stir until all noodles are completely coated. Dish out pasta and pour remaining sauce on top. Eat immediately. (The sauce gets clumpy when cold. The best way to reheat is on the stovetop, adding a little milk to smooth out the sauce again.)
Notes
For a smoother sauce, I recommend using block cheese and grating yourself instead of using packaged shredded cheese.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.