01 - Divide the ground beef into 4 equal balls, handling the meat minimally to preserve texture.
02 - Preheat a cast iron skillet or heavy-bottomed griddle over high heat until smoking hot.
03 - Butter the cut sides of the buns and toast on the skillet until golden brown. Set aside.
04 - Place beef balls onto the hot surface. Press firmly with a sturdy spatula to flatten each into a thin patty, approximately ½ inch thick.
05 - Season each patty with salt and pepper. Cook undisturbed for 2 to 2½ minutes until edges develop a deep brown crust.
06 - Flip the patties and immediately top each with a slice of cheese. Cook for 1 to 1½ minutes until cheese melts completely and patties reach desired doneness.
07 - Combine mayonnaise and mustard in a small bowl. Mix until smooth and spread evenly on both sides of each toasted bun.
08 - Layer each burger with bottom bun, lettuce, tomato, cheese-topped patty, onion slices, pickles, and top bun.
09 - Serve hot while patties remain juicy and cheese is melted.