Boil large pot of water; add salt and cook macaroni according to package until just tender. Drain thoroughly.

Meanwhile, stir Velveeta, milk, and butter in saucepan over low 4 to 5 minutes until smooth.

Add mustard and pepper; keep heat low without boiling.

Remove from heat; melt in cheddar, then fold in macaroni.

Stand 1 minute; restir and serve while glossy and flowing.