Diner-Style Double Smash Burgers

Method

1. Stir the mayonnaise, ketchup, mustard, chopped pickles and a pinch of pepper into a sauce and chill it. Divide the cold beef into 16 loose balls of about 2½ oz each without packing them, and keep them in the fridge until the second you cook (you can portion a day ahead, covered).
2. Spread the softened butter on the cut sides of the buns and toast them in batches in a skillet over medium heat for 1 to 2 minutes per batch, until deep golden. Wrap the buns in foil to keep them warm.
3. Heat a large griddle, or two 12-inch cast-iron skillets, over high heat for 5 minutes, until a drop of water skitters, then wipe with a little of the oil. Turn on the vent fan.
4. Set 4 balls on the griddle (or 2 in each skillet), cover each with a square of parchment and press a stiff metal spatula straight down for 10 seconds, until each patty is about ¼ inch thick with lacy edges. Peel off the parchment at once, then season the tops with about ¼ teaspoon salt and a pinch of pepper each.
5. Cook undisturbed about 2 minutes, until the edges are dark brown and crisp. Scrape hard under each patty with the spatula edge to keep the crust, flip, lay on a slice of cheese and cook 30 seconds more.
6. Stack a second cheesed patty on the first, move the pair to a plate, and repeat with the remaining balls, wiping the pan between batches. Hold the finished pairs on a rack in a 200°F oven, or build each burger as its pair comes off. The beef is done when no pink remains (160°F), which thin patties reach in under 3 minutes.
7. Spread sauce on both buns and build each burger with a double stack, sliced onion and pickle chips.