In separate batches, pulse the chuck and the sirloin in a food processor 10 times. Combine the chuck, sirloin, and kosher salt in a large bowl. Form the meat into 5-ounce patties.
Heat a cast iron skillet or griddle over medium-high heat for 2 to 3 minutes. Place the hamburger patties in the pan. For medium-rare burgers, cook the patties for 4 minutes on each side. For medium burgers, cook the patties for 5 minutes on each side. Flip the burgers only once during cooking.
Recipe courtesy of Alton Brown