I do this all in a non stick pot. I cook the Macaroni as directed on the package then I drain it and put back in pot. While it is still hot I cut up the Velvetta in pieces and stir into hot Macaroni and let melt. Then add the butter.
Then I salt, pepper and add dry mustard. Once the cheese is melted I add the milk and stir it up. Simmer on low heat.
We like to sprinkle with BK seasoning, or a seasoned salt.