Preheat a cast-iron skillet over high heat.
Divide ground beef into two loose balls.
Place one beef ball onto the hot skillet.
Smash firmly within the first 10 seconds of placing the beef using a sturdy spatula (or spatula with parchment) to create a thin patty.
Season with salt and black pepper.
Cook without moving for 2–3 minutes.
Flip once when edges become crispy.
Add cheese slice on top if using.
Cook the second side for 1–2 minutes until the patty is browned and cooked through with juices running clear.
Transfer patties directly to toasted buns to keep edges crisp and juicy.
Toast burger buns lightly on skillet.
Assemble burgers and serve immediately.