01 - Divide the ground beef into 4 equal balls (do not overwork the meat).
02 - Preheat a cast iron skillet or heavy-bottomed griddle over high heat until very hot.
03 - Butter the cut sides of the buns and toast them on the skillet until golden brown; set aside.
04 - Place beef balls onto the hot skillet. Using a sturdy spatula, press down firmly to flatten each ball into a thin patty (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 the patties, immediately top each with a slice of cheese, and cook for another 1-1 1/2 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 - Assemble each burger: bottom bun, lettuce, tomato, smash burger patty with cheese, onion, pickles, top bun.
09 - Serve immediately while hot and juicy.