In a sauce pan melt butter, stir in flour, mustard and salt. Slowly add milk and stir and slowly add cheese until all ingredients are blended together and cheese is melted.
I use a large Pyrex glass measuring bowl or cup and do this in a microwave oven until all ingredients are melted and blended!
Add macaroni and place ingredients in a buttered 8x9 Pyrex dish and bake 25-30 minutes at 375 degrees. Helen Bobe baked until the top was lightly browned!