1Preheat a cast iron skillet or griddle over high heat until it reaches 375-400°F, about 5 minutes. The surface should be smoking hot.
2Cook the bacon strips in a separate skillet over medium heat until deeply crispy and the fat has rendered completely, about 8-10 minutes. Transfer to paper towels and set aside.
3Gently shape the ground beef into a loose ball without overworking the meat. Don't form it into a patty yet - this happens on the griddle.
4Season the beef ball generously on all sides with kosher salt and black pepper immediately before cooking.
5Place the beef ball onto the screaming hot griddle. Immediately press down firmly with a sturdy spatula for 5 seconds to create a thin, wide patty about 1/4-inch thick. Don't press again - this single smash is crucial.
6Cook undisturbed until the edges are deeply browned and crispy, about 2-3 minutes. You'll see the patty developing a dark crust.
7Flip the patty once. Immediately lay the 2 American cheese slices over the top. Cook for another 1-2 minutes until the bottom develops a crust and the cheese begins melting.
8While the burger cooks, spread butter on the cut sides of the bun. Toast the bun cut-side down on the griddle until golden brown with crispy edges, about 1-2 minutes.
9Remove the patty from heat and allow residual heat to finish melting the cheese completely.
10Spread mayonnaise on the top bun and mustard on the bottom bun.
11Layer the crispy bacon strips on the bottom bun, then place the cheese-topped patty directly on the bacon.
12Top with lettuce leaves, tomato slices, pickle rounds, and diced onions in that order.
13Crown with the top bun and serve immediately while everything is still warm and the cheese is melty.