In a large stockpot over a medium flame, melt the butter. Whisk in the flour and continue to cook until a nice tan roux has formed. Try not to let the roux burn.
Remove the stockpot from the heat and add in the milk. Place the stockpot back over the flame and allow it to simmer until thickened – about one minute.
Add the pumpkin puree and shredded cheeses while stirring constantly to evenly mix and melt. Mix in the spices: cayenne, pepper and salt along with the Dijon mustard. Continue to simmer and stir until desired thickness.
While the sauce is simmering, bring a large pot of salted water to a boil. Once boiling, cook the pasta as directed – approximately 10 minutes. Drain the pasta and place in a large mixing bowl. Gently fold the sauce into the pasta to evenly coat. Serve warm.
Note I have to say this was yummier the next day, I threw the leftovers in a baking dish and sprinkled the top with whole wheat Panko bread crumbs and baked it for about 15 minutes at 350°F – it was delicious!