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