1Preheat a cast iron skillet or flat-top griddle over high heat until smoking hot, about 5-7 minutes.
2Cook bacon strips in the hot skillet until crispy but still pliable, turning occasionally, about 6-8 minutes. Transfer to a paper towel-lined plate and set aside.
3Form ground beef gently into a loose ball without overworking the meat. Keep the ball slightly larger than a golf ball for proper smashing.
4Place beef ball onto the screaming hot cooking surface. Immediately press down firmly with a sturdy metal spatula, smashing the patty thin and flat against the griddle for 5-10 seconds. This creates the signature crispy, caramelized crust.
5Season the exposed surface generously with kosher salt and freshly ground black pepper while the first side cooks undisturbed for 2-3 minutes until deeply browned and crispy at the edges.
6Flip patty once using the spatula, scraping up all the caramelized bits. Season the cooked side with additional salt and pepper. Cook second side for 1-2 minutes until cooked through.
7Place American cheese slice on top of patty immediately after flipping so it melts from the residual heat.
8While patty finishes cooking, add butter to a clean section of the griddle. Split hamburger bun and place cut-sides down in the melted butter. Toast until golden brown and crispy, about 1-2 minutes.
9Remove patty from heat and layer bacon strips on top of the melted cheese.
10Assemble burger by placing the cheesy bacon-topped patty on the toasted bottom bun. Add any desired toppings in this order: mayonnaise on top bun, then lettuce, tomato slices, pickles, and onions.
11Cap with the toasted top bun and serve immediately while piping hot.