Divide cold ground beef into 3 equal portions (about 5.5 oz each). Handle as little as possible. Form into rounds about 1 inch wider than your bun. Press a shallow dimple into the center of each with your thumb. Refrigerate until ready to cook.
Season
Season both sides generously with kosher salt and cracked black pepper right before cooking.
Toast the Buns
Heat a large cast iron over medium heat. Butter the cut sides of each brioche bun and press face-down into the skillet until deep golden, 1 to 2 minutes. Set aside.
Sear the Patties
Increase heat to medium-high until the skillet is barely smoking. Add patties and cook undisturbed for 3 to 4 minutes until a deep mahogany crust forms and the patty releases cleanly from the pan.
Flip and Melt the Brie
Flip each patty. Top immediately with 2 to 3 thick slices of Brie. Add 1 tsp water to the pan and cover with a lid or inverted metal bowl. Steam for 2 to 3 minutes until the Brie is fully melted and the internal temperature reads 160°F.
Build and Serve
Spread mayo on the bottom bun if using. Add a handful of arugula. Place the Brie-topped patty on top. Spoon warm bacon jam generously over the cheese. Close with the top bun and serve immediately.
Notes
80/20 ground beef is non-negotiable. Leaner blends make dry patties.
Keep formed patties cold until they hit the pan - cold fat on hot iron builds the crust.
Brie melts fast. Set a timer for 2 minutes once the lid goes on.
Warm the bacon jam before topping - cold jam seizes and won't coat properly.
Storage: Cooked patties keep in the fridge up to 3 days. Store separately from toppings. Reheat in a skillet, not the microwave. Raw patties freeze up to 1 month.