Preheat oven to 375°F. In large skillet, combine ground beef, onions, bell peppers and chili powder. Cook over medium heat until meat is brown. Add mushrooms; simmer 3 minutes, stirring with wooden or plastic spoon to prevent sticking. Transfer beef mixture to 9 inch pie plate. Pour corn evenly over beef mixture.
In large bowl, combine sour cream, salt, pepper and red pepper sauce. Spread evenly over corn. Spread mashed potatoes over sour cream layer. Dot with pieces of butter; top evenly with shredded cheese.
Cover and bake until heated through, about 45 minutes. Garnish with chopped scallion.