Cook the bacon. In a skillet over medium heat, cook bacon until crisp. Transfer to paper towels and let cool. Reserve 1 teaspoon of the bacon fat for the baconnaise. Finely chop the bacon.

Make the baconnaise. In a small bowl, mix mayo, Dijon, lemon juice or pickle brine, smoked paprika, garlic, black pepper, chopped bacon, and the reserved bacon fat. Stir until smooth. Taste and adjust salt or acidity. Chill until ready to use.

Prep the beef. Divide ground beef into 8 equal portions (about 3 ounces each). Roll each into a loose ball—don’t compact it. Season the tops lightly with salt and pepper.

Heat the surface. Set a cast-iron skillet or griddle over medium-high heat until it’s ripping hot. Lightly oil the surface. You should see a faint shimmer and a hint of smoke.

Toast the buns. Spread butter on the cut sides of the buns. Toast face-down in the skillet until golden. Set aside.

Smash the patties. Place two beef balls in the pan, spaced apart. Immediately smash each with a sturdy spatula (or two spatulas) to about 1/4-inch thick. Press firmly for 10–15 seconds to lock in maximum contact.

Season and sear. Sprinkle a little more salt and pepper on the raw side if needed. If using onions, scatter a few thin slices on top of each patty so they fuse into the meat.

Flip and cheese. When the edges are deeply browned and lacy (about 60–90 seconds), scrape under each patty with a firm spatula and flip. Add cheese to each, then cook 30–45 seconds more until the cheese melts and the patty is just cooked through.

Repeat. Transfer patties to a warm plate. Repeat with remaining meat, adding a touch more oil if needed. Work in batches so you don’t crowd the pan.

Build the burgers. Spread baconnaise on both sides of each bun. Stack two cheesy patties on each bottom bun. Add pickles and lettuce if you like. Cap with the top bun.

Serve right away. Smash burgers are best hot off the skillet when the edges are still crackly. Eat immediately.