Place the bottoms of the buns on a work surface, cut-side up. Top with the salami, fontina, and pepperoncini (if desired).
Sandwich with the tops of the buns. Flatten slightly with the back of a spatula. Spread the top of each sandwich with ½ tablespoon of the butter.
Heat a large skillet over medium heat. Cook the sandwiches, buttered-side down, pressing occasionally with the back of a spatula, for 2 minutes.
Flip and cook until the cheese melts and the bread is golden.