Cook the noodles in a large pot according to package directions.
While the noodles cook, the your butter to a warm pot and stir it continuously. You do not want it to burn or brown.
Once the butter is melted immediately add flour and make a roux. Whisk continuously until well blended. This will make the base for the mac and cheese so it's really important. You don't want it to be grainy still.
Once the Roux is done add the milk and heavy cream. Simmer. DO NOT BRING TO A BOIL.
Add the cheese and stir continuously over a low flame until it is all melted and combined.
Add the salt, Dijon mustard and ground mustard. Stir in well.
Add additional salt and pepper to taste.
Once the noodles are done, drain them well and then add them to the cheese mix.
Serve immediately.