Boil elbows in highly salted water. Drain. While elbows are boiling, in large saucepan, heat butter and cheese until melted. Add 4 cups buttermilk, dry mustard and eggs. Heat until thoroughly melted and blended. Add to drained elbows. Stir together. Pepper to taste. Add more buttermilk to just the top of elbows. Put 4 or 5 slices of American Cheese on top. Bake uncovered in a 350 degree preheated oven for 55 minutes or until hot and bubbly.
NOTE: Even if you hate buttermilk you will love this. The buttermilk just gives it that sharp pungent taste that you crave in macaroni and cheese dishes.