Preheat oven to 350º F. Butter an 8×8-inch baking pan with 1 tablespoon butter; set aside. Cook and drain macaroni according to package directions; set aside. Melt 2 tablespoons butter in a medium saucepan over medium heat. Whisk in flour, mustard, salt and pepper; cook 2 minutes, stirring frequently. Whisk in milk and cook, stirring frequently, until mixture thickens; about 10 minutes. Stir in cheese until cheese is melted and sauce is smooth. Stir macaroni into sauce; pour macaroni mixture into prepared pan. Melt remaining tablespoon of butter and toss with popcorn and parsley flakes. Spread popcorn over macaroni and bake 10 minutes.