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