Set the slow cooker to low.
Cook and drain macaroni.
In a saucepan over low heat, melt the butter and cheese; remove from heat.
Add eggs, sour cream, soup, salt, milk, mustard and pepper. Stir well.
Add drained macaroni and stir.
Pour into preheated slow cooker. Cook for 3 hours.
**NOTE: You can add additional cheese on top when there's 10 minutes left for cooking.
**VARIATION: This can be cooked in the oven, covered at 350 degrees F for 45 minutes.