Cook macaroni noodles al dente, according to package instructions. Rinse with cold water to stop the cooking process. Drain completely.
In a large saucepan, melt butter over medium-low heat.
Stir in flour; continue cooking over medium-low heat, stirring constantly, until mixture begins to turn golden. Stir in mustard powder, salt, and pepper.
Slowly add milk. Cook, stirring constantly, over medium heat until thickened to the consistency of gravy and bubbly.
Remove from heat; add cheese and stir until melted.
Mix cheese mixture with cooked macaroni; pour into greased 9x13 inch casserole dish.
Combine bread crumbs and melted butter; sprinkle over mac & cheese.
Bake at 375 degrees 20-30 minutes or until bubbly.
Remove from oven; top with crumbled bacon, chives, and sour cream.