Divide the ground beef into 8 equal portions. Roll each portion into balls and place on a plate. Cover and refrigerate for at least 20 minutes or up to 6 hours.
Heat a large cast iron skillet or griddle over medium-high heat. If desired, toast the buns by brushing the cut side of the buns with melted butter and placing on the griddle until golden brown, about 1 minute.
Increase heat to high. Place 2 to 4 balls of ground beef on the very hot griddle (leave plenty of room between each, so cook fewer at a time if needed). Smash the beef with a spatula into thin patties. Season the top of each patty with salt and pepper.
Cook until browned and crisp, about 2 minutes. Flip and season the cooked side with salt and paper. Cook until crisp and browned on the bottom, about 1 minute more.
Top half of the patties with a cheese slice and place a plain patty on top of the cheese. Remove from skillet or griddle. Repeat with the remaining beef and cheese. Serve the double patties on buns with your favorite toppings.
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Notes
Don’t skip chilling the burger patties after they are shaped. The cold meat hitting the hot griddle helps to form the exterior crust. It also helps prevent the center from overcooking. Because the skillet is very hot and the patties are thin, the colder internal temperature will take longer to rise and longer to fully cook, allowing the outside to crisp up without overcooking the center for juicy burgers.
If the patty sticks to your spatula when smashing, place a piece of parchment paper on top of the patty before smashing it. You can also do this if your spatula has slots in it.