How to Make It
Wedge the thyme sprigs into the hinge between the bones and meat of the rib roast. Season the roast all over with the 1/4 cup of salt and place bone side down on a plate; refrigerate uncovered overnight.
Set the roast in a roasting pan and let stand at room temperature for 2 hours. Preheat the oven to 225°. Cook the roast until an instant-read thermometer inserted in the thickest part registers 90°, about 2 hours. Remove the pan from the oven and increase the oven temperature to 500°. Once the oven is at 500°, transfer the roast back to the oven and cook until golden brown and 110° to 115° for rare, 15 to 30 minutes. (Alternatively, you can let the roast rest at room temperature for 2 hours before roasting at 500°; it will take 30 to 40 minutes to reach 110°.) Transfer the roast to a carving board. Dot with the butter, tent with foil and let stand for 30 minutes.
Meanwhile, set the roasting pan over 2 burners over moderately high heat. Add the shallots and cook, stirring, for 1 minute. Add the capers and cook, stirring, for 30 seconds. Add the Cognac, then the beef stock. Bring to a boil and cook for 5 minutes. Stir in the cream, season with salt and pepper and pour the gravy into a pitcher or gravy boat. Discard the string and thyme sprigs from the roast, carve the meat and serve.