How to Make It
Set up a plancha for cooking (see Note for technique).
While plancha heats, divide beef into 4 equal portions and roll between palms to form loosely packed balls.
Lay beef balls on plancha, leaving 4 inches between balls. Sear 1 minute. Use the back of a large spatula to press burgers firmly, smashing them into 1/4-inch-thick patties. Sear until almost cooked through, about 3 minutes. Sprinkle generously with salt and pepper, and flip burgers. Cook burgers 1 minute; remove to a plate to rest.
Place bacon slices on plancha and cook until crisp and fat is rendered, about 2 minutes and 30 seconds per side. Transfer to plate with burgers. Cut bacon strips in half crosswise.
Spread mayonnaise on 4 bread slices, and top each with 2 tomato slices and several lettuce leaves. Place a burger and 2 bacon pieces on each, and cover with remaining 4 bread slices. Cut into triangles and serve immediately.