Heat oven to 200ºF.
Sprinkle steaks with kosher salt and crushed black pepper. Melt the butter in a 12-inch, heavy-bottomed sauté pan over medium heat. Once butter is hot, add the steaks and cook until browned on both sides, 2 minutes per side for medium-rare and 3 minutes per side for medium. Remove steaks from pan and place on a wire rack set on a half sheet pan; place in oven to keep warm.
Add the stock to the sauté pan and whisk until the crispy bits release from bottom of pan. Reduce the liquid for 3 to 4 minutes over medium-high heat. Add the cream, cognac, and green peppercorns to the pan. Increase the heat to high and cook, whisking constantly, until sauce thickens slightly, just enough to coat the back of a spoon, 5 to 7 minutes.
Season the sauce to taste with kosher salt. Place steaks on plates, top with sauce, and serve immediately.