Divide ground beef into 4 equal portions (about 4 oz each). Roll each into a loose ball—do not pack tightly. Season with a little salt and pepper.
Heat a skillet or griddle over medium heat. Spread a bit of butter on the cut side of each bun and toast until golden brown. Set aside.
Increase heat to high. Place beef balls on the hot surface, leaving space between them. Using a heavy spatula or burger press, firmly smash each ball down into a thin patty. Hold for 10 seconds to ensure even contact. Season again with salt and pepper.
Cook patties for 2–3 minutes or until crispy and brown on the bottom. Flip, add a slice of cheese to each patty, and cook another 1–2 minutes until cheese is melted and burgers are cooked through.
Spread sauce on the bottom bun, add the patty, and layer with pickles, onions, lettuce, and tomato as desired. Top with the other half of the bun.