01 - Divide the ground beef into 4 equal balls, handling the meat minimally to maintain texture. Do not overwork or compact the beef.
02 - Preheat a cast iron skillet or heavy-bottomed griddle over high heat until the surface is very hot and smoking slightly.
03 - Butter the cut sides of the buns. Place them butter-side down on the hot skillet and toast until golden brown and crispy. Remove and set aside.
04 - Place beef balls onto the hot skillet. Using a sturdy metal spatula, press down firmly to flatten each ball into a thin patty, approximately 1/2 inch thick. Season immediately with salt and pepper.
05 - Cook the patties undisturbed for 2 to 2.5 minutes until the edges are browned and crispy, developing a dark crust.
06 - Flip each patty and immediately top with a slice of cheese. Cook for another 1 to 1.5 minutes until the cheese is fully melted and the patty is cooked through.
07 - In a small bowl, whisk together the mayonnaise and yellow mustard until fully combined.
08 - Spread the mustard-mayonnaise mixture on both sides of each toasted bun. Layer from bottom to top: bun, lettuce, tomato slices, cheese-topped patty, red onion, pickle slices, and top bun.
09 - Serve the smash burgers immediately while hot and juicy to enjoy optimal texture and flavor.