Divide beef into 8 loose 3-ounce balls (weigh if possible) and keep chilled.
In a small bowl, whisk together all sauce ingredients. Chill until ready to use. Heat a large electric or flat-top griddle to 400°F.
Place 2 beef balls on the hot surface. Place a square of parchment over each and press firmly with a burger press, to about 1/4th-inch thin. Hold for 10 seconds, then peel off the parchment and discard. Season with salt and pepper.
Cook for 60 to 90 seconds, until the edges are deeply browned and juices bubble up. Use a sharp metal turner to scrape under and flip (see note 1). Season the other side with salt and pepper. Top with a slice of cheese and cover with a pot lid (see note 2) to melt the cheese and cook for another 30 to 45 seconds. Transfer to a plate. If making a double-patty burger, stack two patties together while hot.
While the patties cook, butter the buns and toast them cut-side down on a cooler area of the griddle until golden.
Spread burger sauce on both halves of the toasted buns. Layer with lettuce, tomato, one or two patties, and avocado or pickles if using. Serve immediately; these are best hot off the griddle!
Video
Notes
Note 1: If the patty breaks apart when flipping, that’s totally fine. Just press it back together as best you can and add the cheese. The cheese on top helps hold everything in place.Note 2: Covering the patties with a large pot lid while the cheese melts helps it get extra gooey and fully melted.Storage: Store cooked patties in an airtight container in the fridge for up to 3 days. Reheat in a hot skillet until warmed through. Store sauce in the fridge for up to 5 days. Assemble burgers fresh for best texture.