Cook macaroni until al dente. Drain.
While pasta is cooking, mix eggs, milk, mustard, salt, and pepper. Whisk well.
When macaroni is done, return to hot pot.
Turn heat to LOW. Add butter. Toss and stir to coat.
Add in egg mixture. Mix well.
Add cheese in several batches, stirring constantly until cheese is melted and creamy, about 3-4 minutes.