Place the meat on a rack inside a roasting pan to elevate it.

Combine Dijon mustard, grainy mustard, and peppercorns in a bowl to form a paste.

Spread the paste over the prime rib roast, fat side up.

Marinate the roast in the refrigerator overnight, if desired.

Combine flour and salt in a medium bowl for the Yorkshire pudding.

Whisk in the eggs, water, and refrigerate the mixture.

Preheat oven to 350 degrees F.

Remove the prime rib from the refrigerator about a half hour before cooking and season with salt.

Place the meat in the oven and cook for 12 to 15 minutes per pound until the meat reaches an internal temperature of 135 degrees F.

Remove the Yorkshire pudding mixture from the refrigerator when the roast is nearly finished.

Remove the roast from the oven and let it rest for at least 20 minutes before slicing.

Raise the oven temperature to 400 degrees F and place the baking dish for the Yorkshire pudding inside.

Whisk warm milk and some of the melted butter into the batter.

Remove the hot baking dish from the oven and add the remaining melted butter.

Pour the batter over the butter and bake in the oven until golden brown and puffy, about 30 minutes.

Cut the Yorkshire pudding into 8 wedges and serve immediately with a slice of prime rib and pan drippings.