Melt 2 tablespoons butter in a large saucepan over medium heat, and whisk in all-purpose flour; cook, whisking constantly, 2 minutes.
Whisk in milk gradually; cook, whisking constantly, 5 minutes or until mixture thickens. Remove from heat. Add shredded cheese, salt, and pepper, stirring until cheese melts. Stir in broccoli and pasta. Pour into a lightly greased 11- x 7-inch baking dish. Sprinkle with breadcrumbs; dot with remaining 2 tablespoons butter.
Bake at 350° for 25 to 30 minutes.
To lighten: Substitute equal portions reduced-calorie margarine, 2% reduced-fat milk, and 2% reduced-fat shredded sharp Cheddar cheese, and reduce salt to 1/4 teaspoon. Cook as directed, and pour into baking dish. Stir together remaining margarine, melted, and breadcrumbs; sprinkle over pasta mixture. Bake as directed, and let stand 5 minutes before serving.
Stir in chopped chicken if you want some meat in this dish.