Add the chicken breasts, mild salsa, and frozen corn to the slow cooker. Stir once to combine the salsa and corn, then cover and cook on low for 6 hours.
Remove the chicken breasts and shred them with two forks. Return the shredded chicken to the slow cooker and stir it into the salsa and corn mixture until evenly coated.
Heat 1 tablespoon of butter in a large skillet over medium heat. Place one tortilla in the skillet, sprinkle 1/4 cup cheese over half of it, then top with about 1/2 cup of the chicken mixture and another 1/4 cup cheese.
Fold the tortilla in half and cook for 2 to 3 minutes until the bottom is golden. Flip and cook the other side until golden and the cheese melts. Repeat with the remaining butter, tortillas, cheese, and chicken mixture.