Set aside 4 slices of bacon for topping, and finely chop the remaining 4 slices to mix into the ground beef.
Add 2 egg yolks, salt, and pepper to the minced beef, gently mixing until just combined without making the mixture too firm. Refrigerate for 30 minutes to rest.
Lightly oil your hands, then shape the mixture into rounds, placing a piece of blue cheese in the center and wrapping it completely. Gently press the center to prevent puffing during cooking.
Heat a pan and add a small amount of oil. Cook the reserved bacon slices until crispy, remove and set aside. Melt butter in the same pan and toast the cut side of the hamburger buns until golden brown.
Place the patties on the pan and adjust heat to cook one side thoroughly before flipping.
When the cheese inside the patty begins to melt, place mozzarella and cheddar cheese on top, followed by the cooked bacon and a layer of Emmental cheese. Cover with a lid to fully melt the cheese.
Place the melted-cheese patty and bacon on the bottom half of the toasted bun.
Stack the top half of the bun with sliced tomato (lightly sprinkled with salt and pepper), fresh onion, pickles, and lettuce in order.
Spread mayonnaise and yellow mustard on the top bun, then close the burger to finish.
Mix chopped bacon, egg yolk, salt, and pepper into the beef to form the patty dough, then refrigerate for 30 minutes.
Shape the patty with blue cheese in the center, and toast the bun and reserved bacon in the pan.
Cook the patty, flip it, then add mozzarella, cheddar, bacon, and Emmental cheese, covering with a lid to melt.
Layer the patty and prepared vegetables on the bun, then top with sauce-covered bun to complete.
Cooking tips 💡
Avoid over-mixing the beef mixture, as excessive handling removes air pockets and results in a dense, rubbery texture like fish cake—keep mixing light to preserve juiciness.
Do not frequently move or press the patty after placing it in the pan; wait until the bottom is firmly cooked to avoid breaking or tearing.
Sprinkling a little salt and pepper on the sliced tomato enhances the overall flavor of the burger significantly.