Heat the chicken, salsa, beans, 1/2 cup of the broth and the chili powder in a 10-inch cast-iron skillet over medium heat, stirring, until simmering. Stir in the scallions and remove from the heat.
Meanwhile, combine the cornmeal with the remaining 1 cup broth and 1 cup water in a medium pan. Bring to a simmer over medium heat, stirring, until very thick, 5 to 7 minutes. Remove from the heat and stir in the cheese and butter. Season with 1/4 teaspoon salt and 1/4 teaspoon pepper.
Spread the cornmeal mixture over the filling and bake until cooked through, about 30 minutes. Let stand for 15 minutes. Serve with sour cream.