These incredibly juicy burgers are topped with cheddar cheese.
Use a fork to combine the chuck, sirloin, steak sauce, egg yolks, salt, and pepper in a large bowl; do not mash mixture. Divide into 10 equal parts. Form patties. Make an indentation in each patty; place a piece of butter inside, and encase completely with meat.
Heat a grill, grill pan, or broiler. Cook patties 5 minutes, flip, and top with cheese. Cook 3 to 5 minutes more, until almost cooked through. Transfer to a plate; cover with foil. Let rest for 5 minutes before serving.