Heat a large cast-iron skillet over medium-high heat for 20 minutes.
For the steakhouse spice mix: Mix together the peppercorns, garlic, onion, parsley, dry mustard and red pepper flakes in a small bowl.
For the burgers: Gently toss together the chuck and sirloin in a large bowl, making sure not to overwork the mixture. Form the beef into four 3/4-inch-thick patties, each about 4 inches in diameter. Sprinkle them generously with salt and a few grinds of pepper.
Spread the steakhouse spice mix evenly on a plate. Coat both sides of the patties with the mixture.
Place the patties in the hot skillet; do not press down with a spatula. Cook until the patties are seared and browned on one side, about 3 1/2 minutes, then flip and top each patty with one-quarter of the butter. Cook on the second side until browned and the burgers are medium-rare, about 3 1/2 minutes more.
Assemble the burgers using your favorite buns, toppings and condiments.
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