2 pounds ground chuck
1 pound ground sirloin
3 tablespoons steak sauce (recommended: Crosse & Blackwell)
6 extra-large egg yolks
1 1/2 teaspoons kosher salt
3/4 teaspoon freshly ground black pepper
4 tablespoons cold unsalted butter
12 hamburger buns
Carefully mix the meats, steak sauce, egg yolks, salt, and pepper with the tines of a fork, but do not mash it. Lightly form each hamburger and lightly press into a patty shape. Make an indentation and put a thin slice of butter inside each hamburger, making sure the butter is entirely encased in the meat.
Heat a grill, broiler, or saute pan and cook the hamburgers for 3 to 5 minutes on each side until almost done. Remove to a plate and cover with aluminum foil. Allow the hamburgers to rest for 5 minutes and serve immediately on buns.