Combine chili powder and salt; rub over both sides of chicken. Place chicken on a grill rack or broiler pan coated with cooking spray; cook 5 minutes on each side or until done. Cool slightly, and coarsely chop.
Preheat oven to 350°.
Place tortillas directly on middle rack in oven. Bake at 350° for 7 minutes (tortillas should be slightly pliable). Mash beans in a microwave-safe bowl to desired consistency; stir in Dried-Chile Purée. Microwave at high 30 seconds or until hot. Divide bean mixture evenly among tortillas; top with chicken. Spoon 2 tablespoons Tomatillo Purée over chicken; sprinkle each with about 1 tablespoon onions.