Special equipment: One 9-inch round baking dish
For the prime rib: In a medium bowl, combine the mustards and peppercorns. Stir to blend until it forms a paste. Place the prime rib in the roasting pan (fitted with a rack) you intend to cook it in. Place the roast, fat side up, and spread the peppercorn paste over the whole top. If marinating, refrigerate overnight.
For the Yorkshire pudding: In a medium bowl, combine the flour and salt. Stir to blend. Whisk in the eggs, one by one, and then the water. Refrigerate.
For the prime rib: Preheat the oven to 350 degrees F.
Remove the prime rib from the refrigerator about a half hour before cooking. This will allow the meat to come closer to room temperature and help the meat cook more evenly. Season the roast with salt. When the oven is hot, place the meat in the center of the oven. Allow it to cook 12 to 15 minutes per pound (2 1/2 to 3 hours). Cook until the meat reaches an internal temperature of 135 degrees F. When the roast is close to finished, remove the Yorkshire pudding mixture from the refrigerator to allow it to come close to room temperature. Then, remove the pan from the oven and allow the meat to rest at least 20 minutes before slicing.
To finish the Yorkshire pudding: Raise the temperature of the oven to 400 degrees F and place the baking dish for the Yorkshire pudding inside.
Whisk all of the warm milk and some of the melted butter into the batter. When the baking dish is hot, carefully remove it from the oven and quickly add the remaining melted butter to the dish. Pour the batter over the butter and place the dish in the center of the oven. Bake in the oven until golden brown and puffy, 30 minutes. Cut into 8 wedges and serve immediately with a slice of prime rib and some of the pan drippings.
It is essential to place the meat on a rack inside a roasting pan so the meat is elevated off the bottom of the pan. This will allow the heat in the oven to circulate all around the meat as it cooks. I count about 15 minutes per pound at 350 degrees F for the meat to cook. I cook it until it achieves an internal temperature of about 135 degrees F. So juicy! The Yorkshire pudding, which is traditionally made with beef drippings, tastes better to me made with unsalted butter.