Divide the ground beef into 8 equal portions, about 3 ounces (85 g) each. Roll them loosely into balls. Place the beef balls in the refrigerator to keep them cold while you prepare the other ingredients.
Stir the mayonnaise and yellow mustard together in a small bowl to make the burger sauce. Set aside.
Spread the softened butter on the cut sides of the brioche buns. Toast the buns face down in a large skillet or on a griddle over medium heat until golden brown. Set the buns aside.
Heat a griddle or large cast iron skillet over medium-high heat. Place 2 to 4 beef balls onto the hot surface, leaving space between them.
Place a square of parchment paper over each ball. Use a heavy spatula or burger press to push straight down, flattening the meat into a thin patty. Remove the parchment paper.
Season the patties with the kosher salt, black pepper, and garlic powder. Spread a small dab of the burger sauce on the raw side of each patty.
Cook for 2 minutes, until the edges are browned and juices appear on top. Use a metal spatula to scrape firmly under the patty and flip it.
Cook for 1 more minute. The internal temperature of the ground beef must reach 160°F (71°C). Place a slice of cheddar cheese on half of the patties, then stack a second patty on top of the cheese. Remove from the heat.
Spread the remaining burger sauce on the bottom buns. Layer with the pickles, shredded lettuce, tomato slices, and red onion. Add the double meat patties and cover with the top buns.