Preheat oven to 450 degrees Fahrenheit. In a spice or coffee grinder, or with a mortar and pestle, grind nibs until broken into smaller bits. Mix with remaining ingredients except pork and oil.
Trim off the pork's silvery skin, then rinse and pat dry with a paper towel. Lightly coat all sides with vegetable oil. Rub with a generous amount of the cocoa spice rub.
In an ovenproof skillet, heat 1 tablespoon oil over medium-high heat. Add pork and sear until golden brown on all sides, about 8 minutes. Transfer skillet to oven and roast pork until a thermometer inserted into thickest part of meat registers 145 degrees Fahrenheit, about 15 minutes. Let rest 10 minutes before slicing and serving with mango salsa (optional).