Preheat the oven to 350°F. Spray a 13 x 9-inch baking dish with cooking spray. Set aside.
In a medium-size mixing bowl, mix together the chopped chicken, tomatoes, taco seasoning and garlic salt. Set aside.
Drizzle 1/4 cup taco sauce onto the bottom of the dish. Arrange 6 tostada shells over the sauce overlapping where necessary.
In a small mixing bowl whisk together the cream cheese and sour cream. Spread 1/2 over the tostadas.
Top with 1/2 of the chicken mixture, 1/2 cup black beans and 1/2 cup corn. Top with 1 cup shredded cheese. and sprinkle with 1 Tbsp cilantro.
Repeat with 6 tostadas, cream cheese, chicken, black beans and corn. Cover with 1 cup shredded cheese and 1 Tbsp cilantro.
Cover the second layer with 6 tostadas. Pour the taco sauce on top covering completely. Top with remaining shredded cheese. Lay a piece of aluminum foil on top.
Bake for 40 minutes. Uncover and bake for an additional 10 minutes or until the cheese is golden.
Rest for 5 minutes then serve with salsa, sour cream and green onion.