Mix the ingredients together for the Shack sauce and, if serving, bake crinkle cut fries in the oven according to package directions.
Add salt & pepper to ground beef and mix together in a large bowl. Form 4 even round balls of beef.
Meanwhile, heat a large skillet over med-high heat. Add balls, then press down to "smash" into patties. Cook for 3-4 minutes, then flip and cook another 2 minutes. Add 1 cheese slice to each burger, then place lid over the skillet to let the cheese melt and remove from heat.
Remove burger patties from skillet, and wipe the pan clean with a paper towel. Butter the potato buns. Place in skillet, then add back to med-high heat, waiting until buns are lightly toasted, about 2 minutes.
Add burger patties to each bun, then top with tomato, lettuce and Shack sauce. Serve alongside crinkle cut fries if desired, then serve and enjoy!