Before starting the cooking process, I recommend having all of the ingredients weighed, measured, chopped, and laid out and ready to go. It makes the process easier and faster.
Heat oven to 350° F. Spray a 9"x13" baking dish with cooking spray.
Bring a pot of water to a boil and add 1 Tbsp of the salt. Add the pasta and stir. Bring back to a low boil and reduce the heat to maintain low boil. Cook until al dente, then drain and transfer to a large mixing bowl.
Meanwhile, as the pasta is cooking, prepare the cheese sauce. In a medium saucepan, heat one stick (1/2 cup) of butter over medium-low heat. Add the flour and whisk until combined and the mixture starts to bubble.
Add the mustard, heavy cream, milk, 1 1/2 tsp salt, and pepper. Whisk frequently as the mixture heats up, until the mixture thickens.
Add the processed cheese and stir until it is melted. Then add the cheddar cheese and stir until melted and no lumps remain.
Pour the cheese sauce into the bowl of cooked macaroni pasta and stir to coat all of the noodles. Then transfer to the prepared baking dish. Smooth the top.
Melt 4 Tbsp remaining butter and add to a small bowl with the paprika and panko bread crumbs. Stir to coat the bread crumbs with the butter. Then sprinkle the bread crumbs evenly over the macaroni and cheese.
Bake uncovered for 20-30 minutes, or until the top is a golden color and the mac and cheese is bubbling. Try not to over bake so it doesn't dry out.
Remove from oven and let stand for 5-10 minutes before serving.