How to Make It
Heat oven to 425° F. Heat a large skillet (preferably cast iron) over high heat. Season the steak with 1 teaspoon salt and 2 teaspoons pepper.
Add the oil to the skillet and heat for 10 seconds. Add the steaks and cook until brown, 3 to 4 minutes per side. Transfer to oven and cook until an instant-read thermometer inserted into the center of the thickest steak registers 125° F, 4 to 6 minutes. Transfer to a plate. Let rest for 5 minutes, then thinly slice against the grain.
Meanwhile, pour off all but 2 teaspoons of the fat; reserve the skillet. Carefully add the cognac to the reserved skillet. Cook over medium-high heat until reduced by half, 1 to 2 minutes. Add the flour and cook, whisking, for 30 seconds. Slowly whisk in the broth. Cook until reduced by half, 5 to 6 minutes. Add the cream and ¼ teaspoon salt. Cook until thickened, about 30 seconds. Remove the skillet from heat. Add the butter and stir to combine.
Serve the warm steak drizzled with the sauce.