To make the Smasher sauce, combine all the ingredients in a small bowl. Mix thoroughly and set aside to let the flavors develop.

Spread softened butter evenly on the cut sides of the potato buns. Place the buns cut-side down on a griddle or skillet and toast until golden brown. Set aside.

Divide the ground beef into 8 equal portions and form each portion into a loose ball.

Heat a large griddle or cast-iron skillet over high heat until it's very hot. Place a beef ball on the hot surface and immediately smash it very thin with a spatula or burger press. Season generously with salt and pepper. Cook for 1 to 2 minutes until the edges become crispy. Flip the patty, season the other side with salt and pepper, and top each patty with a slice of cheese. Cook for another 30 to 45 seconds until the cheese begins to melt. Repeat this process with the remaining patties.

To assemble each burger, place a bottom bun on the plate and spread it with Smasher sauce. Add 2 cheese-topped patties followed by 4 pickle slices and a tablespoon of onions. Spread additional Smasher sauce on the top bun and place it on the burger. Serve immediately while hot.