How to Make It
In a food processor, combine the salt, pepper, garlic cloves, rosemary and chile powder and process until fine. Add the olive oil and pulse to form a paste. Place the prime rib roast on a cutting board, bone-side up and rub with 1 tablespoon of the salt paste. Transfer the meat to a large roasting pan and pack the salt paste all over the fatty surface, pressing to help it adhere. Let the prime rib stand at room temperature for 1 hour.
Preheat the oven to 450°. Roast the prime rib for 1 hour, until the crust is slightly darkened. Lower the oven temperature to 300° and roast for about 2 hours and 15 minutes longer, until an instant-read thermometer inserted into the center of the roast (not touching the bone) registers 135°. Transfer the roast to a large carving board and let the meat rest for 30 minutes.
Carefully lift the salt crust off the meat and transfer to a bowl. Brush away any excess salt. To remove the roast in one piece while keeping the rib rack intact, run a long sharp carving knife along the bones, using them as your guide. Leave on 1/2 inch of meat, more if reserving for leftovers. Carve the prime rib roast 1/2 inch thick and serve, passing some of the crumbled salt crust as a condiment.