To make the rub, combine cocoa powder, ancho chili powder, dried chipotle, and ground cumin in a small bowl. Set aside while you season the pork with kosher salt and pepper. Rub the pork with a generous amount of the cocoa chili rub, then tie it in about 6-8 sections with butcher’s twine.
At this point, if you have time, place the rubbed pork loin on a sheet tray with a wire rack overnight or for 6-12 hours.
Heat the oven to 200°F.
Get a large pan or a cast-iron skillet hot on a stovetop over medium-high heat. Add enough canola oil for searing the pork. Lightly sear the pork on all sides and be careful not to let it burn. The cocoa spice rub will turn dark immediately, so do a very quick sear, just enough to get a crust on the pork.
Transfer pork to a baking dish and slow roast in the oven for about 2-3 hours or until an instant-read thermometer reads 145°F.