Prepare mashed potatoes according to package directions. Stir in butter and 1/2 teaspoon salt; set aside.
Cook ground beef, onion, and seasoned pepper in a large skillet over medium-high heat, stirring until beef crumbles and is no longer pink; drain and return to skillet. Add 1 cup cheese, corn, soup, milk, and remaining 1/2 teaspoon salt, stirring well.
Pour beef mixture into a lightly greased 13- x 9-inch baking dish. Spread mashed potatoes evenly over beef mixture.
Bake at 350° for 20 minutes. Sprinkle with remaining 1 cup cheese, and bake 10 more minutes.