Line a baking sheet with parchment paper.
Divide the beef into 8 even portions, about 3 ounces each.
Loosely roll each beef portion into a ball and place them on the prepared baking sheet.
Cover the baking sheet and refrigerate the beef until ready to grill.
In a small bowl, add the mayonnaise and the mustard and stir to combine. Set the sauce aside.
In a skillet or frying pan over medium heat, melt the butter.
Add the burger buns, cut-side down, to the melted butter and toast until golden on the buttered-side, about 2-4 minutes.
Transfer the buns, cut-side up, to a plate.
Increase the stovetop heat to medium-high.
Place 2-4 of the beef balls onto the hot griddle and firmly smash the burgers straight down using a flat, nonstick spatula.
Season the burger patties evenly with the salt, the pepper, and the garlic powder.
Add about 1/2 teaspoon of the sauce to the top of each of the patties.
Cook the burgers until seared and the juices start to come to the surface, about 2 minutes.
Using a metal spatula, flip the burgers, making sure to get under the caramelized part before doing so, and cook until the reverse side is caramelized, about 1 minute.
Top 1/2 of the cooked patties with 1 slice of the cheese each, then immediately cover each of the cheese-topped patties with 1 of the non-cheese-topped patties.
Transfer the cooked burgers to a clean plate and repeat the cooking process with the remaining patties, the remaining salt, the remaining pepper, the remaining garlic powder, the remaining cheese slices, and most of the remaining sauce (reserving a small portion for the buns).
Evenly smear the remaining sauce across the bottoms of the toasted buns.
Top each of the bottom buns evenly with the lettuce, the tomatoes, the onions, and the pickle pieces.
Place a cooked double-patty on each of the bottom buns over the toppings.
Replace each of the top buns and serve.