Preheat your oven to 450°F (232°C).

In a small bowl, combine the olive oil, kosher salt, cracked black peppercorns, and smoked paprika. Mix well to form a paste.

Rub the spice mixture all over the prime rib roast, ensuring an even coating on all sides.

In a large, oven-safe skillet or roasting pan, add the sliced leeks and spread them out evenly across the bottom.

Place the seasoned prime rib on top of the leeks, bone side down.

Roast the prime rib in the preheated oven for 20 minutes to develop a nice crust.

After 20 minutes, reduce the oven temperature to 325°F (163°C) and continue roasting for about 1.5 to 2 hours, or until the internal temperature reaches your desired doneness (130°F for medium-rare, 140°F for medium).

While the roast is cooking, prepare the black garlic butter. In a medium bowl, combine the softened butter and mashed black garlic. Mix until well incorporated.

Once the prime rib is done, remove it from the oven and let it rest for at least 20 minutes before slicing.

Serve the prime rib slices with a dollop of black garlic butter on top, garnished with fresh parsley and the charred leeks from the roasting pan.