Preheat oven to 350°. Melt 3 tablespoons butter in a large, heavy skillet. Add panko and stir until coated; set aside.
Cook pasta according to package directions, stopping 2 minutes shy of suggested cooking time; drain and set aside.
Meanwhile, melt 4 tablespoons butter in a large, heavy saucepan over low heat; whisk in flour until smooth. Cook, whisking continuously, about 1 minute; don't let mixture brown. Add milk gradually and cook over medium heat, whisking continuously, until mixture is thickened and bubbly, about 5 minutes. Remove from heat; add salt, red-pepper flakes, and cheese, whisking until melted.
Add pasta; then spoon into a lightly greased 2-quart casserole or four lightly greased 2-cup baking dishes. Sprinkle with breadcrumbs. Bake 20 to 25 minutes, or until bubbly.