Prepare the Mac n Cheese: Butter a 2-quart baking dish.
Heat oven to 325º F.
Cook pasta 2 minutes less from package direction, drain and pour into the casserole dish.
While pasta cooks, heat the 4 tablespoons butter in a saucepan over medium heat; add the flour and cook, stirring for about 3 minutes or until you have a light brown roux. Slowly add the milk, stirring constantly until the roux begins to thicken. Add the mustard and a bit of salt and pepper.
Continuing to stir, slowly add the cheese (it helps if you grate the cheese in advance and let it sit at room temperature for about an hour); once the cheese is melted, stir in the sour cream. Mix well, then pour over the macaroni (it will seem very soupy, no worries, it will all work out).
Prepare the Crunchy ToppingI: n a sauté pan, melt the remaining butter and add the panko, stir and sauté until panko is lightly browned. Spoon panko over the top of the mac n cheese and put in the oven for about 30 – 35 minutes, or until bubbly. Let sit 5 minutes and serve.