Butter each burger bun with the softened butter, set aside.
Heat the griddle to high heat (400°F).
Coat a small area with cooking spray or other neutral oil with a smoke point and add the onions, moving around until soft and opaque, with a little char, about 2-3 minutes. Move to the side or off the griddle entirely.
Divide the ground beef into 8 2-ounce balls.
Working in batches (unless you have a large griddle), place 4 of the balls onto the hot surface and immediately smash with a strong, flat spatula. Do not move, just let them sizzle. Cook for about 30-45 seconds. Season from 12 inches above with Kosher salt and freshly ground pepper. Edges should be slightly charred and lacy.
Using the spatula, scrape up the patties and flip. Top with your desired cheese and the sauteed onions, cooking for another 30 seconds.
Remove to a plate.
Immediately, place buttered burger buns down on the hot griddle to char for about 30 seconds or until browned. I actually do this to both sides to get the butter and burger grease effect all over, but that is up to you.
Remove and assemble burgers, 2 patties per burger bun.
Add any desired hamburger condiments.
If you've tried this recipe, come back and let us know how it was in the comments or ratings.
Notes
Do not use lean ground beef. You need a good amount of fat to prevent sticking and get the browned and crispy crust.