Divide the ground beef into 2 equal portions, shape each into a 4-inch (10cm) patty about 3/4 inch (2cm) thick with a thumb indent in the center, then season both sides with the kosher salt and black pepper just before cooking.

Heat a cast iron skillet over high heat for 2 minutes, add the patties to the dry pan, press once lightly, and sear for 3 to 4 minutes until a deep brown crust forms on the bottom.

Flip the patties, lay 1 slice of American cheese on each, and cook for 2 to 3 more minutes until the internal temperature reaches 160°F (71°C) and the cheese is fully melted.

Remove the patties to rest, reduce heat to medium, add the butter to the same pan, and toast the buns cut-side down for about 1 minute until golden.

Spread ketchup on the bottom buns and mustard on the top buns, layer on pickles, onion, the cheesy patty, and optional lettuce and tomato, then press the bun on and serve immediately.