Heat a cast-iron skillet over medium-high heat until screaming hot, about 2-3 minutes. Brush the skillet with vegetable oil.
Working in batches, place 1-2 loose balls of the beef into the hot skillet. Use a spatula to smash each ball of beef into a 1/2'' thick patty. (Don't worry too much about getting a perfectly round patty - we want those craggy edges to crisp up!) Season generously with salt and pepper. Cook undisturbed for 3 minutes, until the outer edges are brown and crisp.
Flip the burger and season with more salt and pepper. Cook for 4 more minutes for medium-well. During the last 30 seconds of cooking, place the cheese slices on top of the patties until melty.
While the burgers are cooking, butter each side of the bun with about 1/2 Tablespoon of the softened butter. Toast under the broiler or in a dry skillet until lightly browned. Set aside.
To assemble, spread 1/2 Tablespoon or more of the secret sauce on each bun. Place each burger patty on a bottom bun and top with lettuce, tomato, onion, and pickles, finishing with the top of each bun. Serve with fries and enjoy!