Heat oven to 400°F. Brush one side of each tortilla with butter. Place one tortilla on ungreased large cookie sheet, butter side down.
In small bowl, mix sour cream and salsa. Spread about 1 1/2 tablespoons of the sour cream mixture over tortilla on cookie sheet. Top each with chicken, 1 tablespoon of the cheese, the corn and onions. Sprinkle with remaining cheese. Place remaining tortilla, butter side up, over filling; press gently.
Bake 15 to 18 minutes or until cheese is melted and top is light golden brown. To serve, cut into wedges. Serve with additional salsa if desired.