Bring ground beef to room temperature and form into balls placing them on separate pieces of parchment paper. Heat cast iron skillet or pan over medium high heat. Add in 1 tsp of oil and 1 tbsp of unsalted butter. Toast the buns for 2-3 minutes, until golden.
Place a ground beef ball onto a hot pan and let it sit for 20 seconds before pressing it down firmly with a burger press. Remove the parchment paper and cook for 2-3 minutes.
While cooking, season the exposed side with salt, pepper, and a drizzle of yellow mustard, if desired. Flip the burger, season the other side with salt and pepper, and cook for another 2 minutes. Add a slice of American cheese, reduce the heat, then add a splash of water and cover to steam the cheese until melted—about 45 seconds. Repeat these steps for the other patties.
For the Big Mac sauce: combine mayo, ketchup, mustard, relish, Sriracha, sugar, smoked paprika, garlic powder, vinegar. Mix until combined. Taste and adjust seasonings if needed.
To assemble: Spread burger sauce on the bottom bun, then layer lettuce, onions, and the first patty. Add the middle bun, then more sauce, lettuce, onions, pickles or relish, the second patty, and finally the top bun with a little more sauce
