Melt butter in 10- inch ovenproof skillet over medium heat. Swirl butter up the sides of the pan to prevent frittata from sticking. Add potatoes to skillet; cook 3 minutes, stirring occasionally.
Beat eggs in medium bowl. Stir in salsa and salt. Stir in 1 cup cheese. Reduce heat to medium. Add egg mixture; stir gently to combine. Cover; cook 6 minutes without stirring or until eggs are set around edges. (center will be wet).
Sprinkle remaining 1 cup cheese evenly over frittata. If skillet is not ovenproof, wrap handle with aluminum foil. Place under preheated broiler 4 to 5 inches from heat source. Broil 2 to 3 minutes or until cheese melted and eggs are set in center. Cut into wedges; serve with sour cream and cilantro.