How to Make It
Preheat a gas grill to medium (350°F to 450°F). Heat a griddle or grill pan on preheated grill until griddle reaches a temperature of 400°F to 425°F. Combine sirloin and chuck in a medium bowl. Grate frozen butter over meat, and loosely mix with your hands to combine.
Lightly grease griddle with oil. Using a 1/2-cup ice-cream scoop, scoop 6 hearty portions of meat mixture onto griddle. Sprinkle burgers evenly with salt and pepper. Cook 2 minutes. Using a spatula, flatten meat mounds into patties. Cook until browned and edges are crisp, about 2 minutes. Turn patties, and cook to desired degree of doneness, 3 to 4 minutes. Place 1 cheese slice on each patty, and cook until cheese melts.
Place patties and desired toppings on potato buns. Wrap burgers in aluminum foil; let stand 5 minutes to allow the bun to steam. Unwrap and discard foil; serve immediately.