Divide the beef into 4 equal portions and roll into loose balls. Refrigerate until ready to cook.
In a small bowl, combine the salt, pepper, garlic powder, and onion powder.
Heat a Blackstone griddle or cast iron skillet over medium heat. Melt 1 tablespoon of butter, add the onions and a pinch of salt, and cook for 6 to 8 minutes, stirring occasionally, until soft and golden. Move to a cooler area to keep warm.
Spread 1 tablespoon of softened butter on the cut sides of the buns and toast until golden, about 1 minute. Set aside.
Increase the heat to medium-high (400°F to 450°F). Place the beef balls on the griddle, cover with parchment, and smash until very thin. Remove the parchment and sprinkle with the seasoning.
Cook without moving for 1½ to 2 minutes, until the edges are crispy and browned.
Flip the patties. Top each with a slice of American cheese and grilled onions.
Place 1 tablespoon of butter (cut into a pat) on top of each burger and let melt for 30 seconds.
Transfer to the toasted buns and serve immediately.