Bring a large pot of water to a boil, add salt and cook macaroni according to the package until just tender. Drain thoroughly.
While the pasta cooks, put Velveeta, milk and butter in a saucepan over low heat. Stir for 4 to 5 minutes until the cubes melt into a smooth sauce.
Stir in mustard and pepper, keeping the heat low enough that the sauce does not boil.
Remove from the heat and stir in the cheddar until melted. Fold in the drained macaroni.
Let stand for 1 minute, stir once more and serve while the sauce is glossy and flowing.