Preheat oven to 400 degrees F. Spray a 9x13 baking dish with cooking spray.
Cook pasta for 6-8 minutes, until al dente.
Meanwhile, melt butter in a large saucepan. Gradually add in flour, whisking constantly. Immediately add salt, ground mustard and black pepper. Stir until well blended, about 1 minute.
Slowly add milk to butter mixture, stirring constantly. Bring to a boil and let boil for about 2 minutes. Lower heat and continue to cook over medium-low heat for 3-5 minutes, stirring constantly.
Slowly add in cheese, reserving 1/4 cup for topping, stirring to combine along the way. Keep stirring until cheese is completely melted.
Remove from heat and add cooked macaroni. Stir until combined.
Add macaroni and cheese to baking dish. Top with leftover 1/4 cup cheese. Bake in the oven for 5-10 minutes or until cheese is melted and bubbly. Allow to cool slightly and serve!
by The Savvy Kitchen