In a small bowl, whisk together the mayonnaise, ketchup, mustard, relish, Worcestershire sauce, and garlic powder. Set aside in the fridge.

Shape the cold ground beef into loose balls, about 4 ounces each. Do not overwork the meat.

Heat a cast iron skillet over medium-high heat until it is very hot. Place a beef ball on the dry skillet and press down firmly for about 10 seconds using a spatula or smash burger press. Season generously with salt and pepper.

Let the patty cook undisturbed for about 2 minutes until the bottom is dark brown and crispy. Scrape under the patty with a sharp spatula to keep the crust intact. Flip it over, add a slice of cheese, and cover with a lid for 30 seconds to melt.

While the meat rests, swipe a bit of butter on your buns and toast them in the burger fat left in the pan.

Spread a generous amount of secret sauce on the bottom bun. Layer with lettuce, onion, and tomato if using. Place your smash burger layers on top and crown with the top bun.