Lightly mix 6 ounces ground beef chuck with a big pinch of kosher salt. Form into two equal balls, and then shape into two flat patties. Lay two slices American cheese between them and form the meat around the cheese; make an indentation in the center of the patty. Heat a cast-iron skillet over medium-high heat; sprinkle the skillet with salt. Cook the burger 4 to 5 minutes per side. Serve on a soft bun.