Preheat oven to 350 degrees.
Cook elbow macaroni according to package directions and set aside.
In a saucepan, heat 1/2 cup butter until melted.
Slowly whisk in flour until blended, and then slowly add in the milk.
Bring mixture to a boil and continue stirring until it starts to thicken.
Add in cheese, mustard, salt, and pepper until smooth.
Add cooked macaroni to the sauce and stir until completely combined.
Pour mixture into a greased 9 x 13 inch baking pan.
Melt the remaining 1/4 cup butter and mix it with the bread crumbs, then sprinkle it over the macaroni mixture.
Bake for about 30 minutes, or until the mixture is heated through.