In a small bowl, mix mustard and black pepper with a tablespoon or more water to make a smooth slurry.
In a small bowl, mix panko and melted butter.
Heat up a large pot (at least 6qt) with water to cook the macaroni. When the water is at a full boil, add 2 tsp salt. Add macaroni and stir. Cook until just tender, 7 to 8 minutes (according to package directions). Stir occasionally while cooking. Drain.
Heat oven to 350 F/180C
To the now empty pot, add 4 tbsp unsalted butter and melt. Add 4 tbsp flour and stir to combine with a wooden spoon or whisk. Keep stirring and cook for about 30 seconds until you can smell the roasted smell of the flour. It will have lost its raw flavor. Do not let the flour brown.
Add milk one cup at a time, stirring in between. Stir and bring to a boil. Stir frequently. When milk has come to a boil, turn off the heat. Add 2 handfuls of cheese and stir until it’s melted into the milk. Keep adding cheese 2 handfuls at a time, stirring until smooth and creamy.
To the sauce, add salt and mustard/black pepper slurry. Add hot sauce if using. Mix well and taste. Add more salt if necessary. Add macaroni and stir well. Taste and adjust salt or hot sauce per your liking.
Spread salted butter all over the bottom and sides of a 9x13 baking dish. Pour macaroni and cheese mixture into the dish. Even out top. Sprinkle breadcrumbs evenly over the top. Bake in the heated oven for 30 minutes or until breadcrumbs are golden. If mac and cheese is already bubbling all over and breadcrumbs are not golden, place pan under broiler for 2 minutes to get golden color on top.