Boil elbow macaroni for about 7 minutes until al dente. Drain well.
In a medium saucepan over low heat, melt butter and 2 cups of shredded cheddar cheese. Stir until smooth.
In the crock pot, combine cheese mixture with beaten eggs, sour cream, cheddar soup, milk, salt, mustard, and pepper.
Stir all ingredients well, then add drained pasta. Mix until fully coated.
Cover and cook on LOW for 2.5 to 3 hours, stirring once or twice during cooking.
Serve hot. For an optional baked version, transfer to a baking dish, sprinkle extra cheese on top, and bake at 350°F for 30 minutes.