1Heat gas or charcoal grill. Between pieces of plastic wrap or waxed paper, place each chicken breast smooth side down; gently pound with flat side of meat mallet or rolling pin until about 1/4 inch thick. Sprinkle with salt and pepper.
2Cut cheese into 4 slices, about 3x1x1/4 inch. Place 1 slice cheese in center of each chicken piece. Roll chicken around cheese, folding in sides. Brush chicken rolls with butter.
3Place chicken rolls, seam sides down, on grill over medium heat. Cover grill; cook about 15 minutes, turning after 10 minutes, until juice of chicken is no longer pink in center. Serve chicken with salsa.