Preheat oven to 350 F. Brown ground beef in a large skillet over medium heat, breaking it up with a spatula as it cooks. Drain grease. Stir in taco seasoning, water, corn, and black beans. Let cook for another 5 minutes.
Spread meat mixture into a greased baking dish. Top with cheese. Place potatoes in a large saucepan and cover with water. Bring to a boil over medium heat. Cook until potatoes are fork tender. Drain and place into a large bowl. Mash potaotes with butter, salt, and pepper.
Spread potatoes over cheese. Bake for 25-30 minutes.
