01 - Divide the ground beef into 4 equal balls without overworking the meat.
02 - Heat a cast iron skillet or heavy-bottomed griddle over high heat until very hot.
03 - Butter the cut sides of the buns and toast on the skillet until golden brown; set aside.
04 - Place beef balls on the hot skillet. Press firmly with a sturdy spatula to flatten each ball into a thin patty about ½ inch thick.
05 - Season each patty with salt and pepper. Cook undisturbed for 2 to 2½ minutes until edges are brown and crispy.
06 - Flip the patties, top each with a slice of cheese, and cook for another 1 to 1½ minutes until cheese is melted and patty is cooked through.
07 - In a small bowl, mix mayonnaise and mustard. Spread on both sides of each toasted bun.
08 - Layer each burger with bottom bun, lettuce, tomato, smash burger patty with cheese, onion, pickles, and top bun.
09 - Serve immediately while hot and juicy.