Remove roast from the refrigerator two hours before roasting to allow the roast to fully come to room temperature. Pat dry. Place oven rack on the second lowest position. Preheat oven to 500°F.

In a small bowl, combine cacao powder, brown sugar, paprika, garlic and onion powders, salt, and pepper. Brush the roast with olive oil and rub the cacao mixture all over the roast. Place the roast in a pan large enough to comfortably hold in, bone-side down. Insert an oven-proof meat thermometer into the thickest part of the roast making sure it is not touching any bone, and roast for 20 minutes. Without opening the oven, reduce the temperature to 325°F and roast for another 13-15 minutes per pound or until the internal temperature of the roast is 120°F for medium-rare. Remove from oven, transfer the roast to a cutting board and cover with foil. Let rest for 20 minutes while making the au jus and sauce. Remove the roast from the bones, transfer to a serving platter, slice, and serve with au jus and horseradish sauce.