Prepare the sauce by combining all sauce ingredients above in a small bowl and stir until fully integrated. (The sauce is even better if sits in the refrigerator for a few hours.)
Prepare toppings and set aside, like slices of tomatoes, onions, pickles, cheese (I prefer American), and lettuce.
To make the burgers, start by dividing the ground beef into four equally sized balls.
Heat a cast iron skillet over medium high heat. Once the skillet is very hot, coat the bottom with a few tablespoons of vegetable oil.
Working one at a time, place each ball of ground beef into the hot skillet. Place parchment square on top of the ball, and working with a burger press or spatula, firmly press down until the patty is very thin.
Season the top of the patty with salt and pepper. If desired, squirt a bit of yellow mustard on top and brush evenly. Sear patty for 2-3 minutes or until the edges of the patty are crispy and caramelized.
Using a spatula, flip the patty onto the other side. Season the other side with salt and pepper. If adding cheese, place a slice on the patty at this point. Continue to sear another 2-3 minutes until cooked through and cheese has slightly melted.
Remove from heat and set aside. Repeat process with remaining patties.
Once patties are cooked, add a bit more oil to the same skillet and place the bun halves face down 1-2 minutes until toasted and golden brown.
Assemble burgers with sauce and desired toppings and enjoy!