Divide 1.5 pounds ground beef 80/20 into 8 balls that are approximately 3 ounces each. Place them in the refrigerator until you are ready to cook them.
Preheat the griddle over medium high heat (approximately 400 degrees F).
Melt 1 tablespoon of the 3 tablespoons butter onto the grill and spread it around with a spatula. Place the 4 hamburger buns on top of the butter on the grill for 2 minutes to toast them, then set them aside while you prepare the burgers.
Place the remaining 2 tablespoons butter on the griddle and move it around to melt it.
Place 4 of the beef balls on the griddle in the melted butter. Top each one with a piece of parchment paper, then firmly press down to smash the beef into thin patties with either a burger press or a large spatula. Discard the parchment paper.
Season the patties with 1/2 teaspoon garlic salt and 1/2 teaspoon pepper. Cook the patties for 2 minutes until the juices in the beef start to rise to the top and the color starts to change.
Cook the patties for 2 minutes until the juices in the beef start to raise to the top and the color starts to change.
Scrape under the patties to get under the caramelized parts and flip them. Once flipped, do not smash the patties again. Cook for 1 more minute and place 1 slice cheddar cheese on 2 of the patties.
Once the cheese is melted, top each patty with cheese with 1 of the patties without cheese to make 2 double patties. Move those patties to a separate platter and continue this process with the remaining 4 beef balls and the remaining 2 slices cheddar cheese until all the patties have been prepared.
Assemble the burgers by placing 4 pieces leaf lettuce, 1 sliced tomato, and 1/2 sliced red onion on the bottom halves of the 4 hamburger buns. Add the double patties on top, then finish with the top buns.
Then the smash burgers are ready to enjoy!