OPTIONAL TOPPINGS: Shredded lettuce, chopped tomatoes, diced red onion, thousand island dressing, ketchup
Pre-heat a dutch oven or large saucepan over medium low heat. Stir 2 tablespoons of butter and flour together to form a paste. Cook for 2-3 minutes or until mixture smells nutty.
Whisk in chicken broth, milk and mustard. Cook for about 10 minutes, stirring often until liquid is thickened completely.
In a small bowl, toss together 2 cups of cheese and cornstarch.
Reduce heat to low, then stir in cheese and cornstarch mixture ½ cup at a time, waiting until cheese is melted before adding more. Cook on low for 5 minutes. Fold in noodles.
Pre-heat oven to 350 degrees. Fold in chopped hamburgers, chopped pickles & bacon crumbles. Spoon mixture into a greased 2 qt. casserole dish. Sprinkle remaining cup of cheese over the top.
In a small bowl, toss together the chopped pretzel bun and the remaining melted butter. Sprinkle coarse salt if desired. Scatter pretzel crumb mixture over the top of the mac & cheese. Bake uncovered for 45 minutes or until mixture is heated through and pretzel topping is golden brown. Remove from oven and serve with assorted mac & cheese toppings on the side.