In a food processor, combine the cocoa powder, kosher salt, cumin, cinnamon, garlic powder, both chilies, almonds and pecans. Pulse until the chilies and nuts are finely chopped. Rub both sides of the sirloin cap roast with the spice rub. Refrigerate for 3 hours or overnight.
Place the sirloin cap on the counter and let stand for 30 minutes, or until it reaches room temperature. Preheat the oven to 200ºF. Place the sirloin cap on a baking sheet and roast until an instant-read thermometer stuck into the thickest part reads 115℉.
Just before the roast is done, heat a large cast iron skillet over medium-high heat until hot. Sear the roast, turning, until a crust forms, about 1½ minutes per side. Transfer to a cutting board, tent with foil and let rest for at least 8 minutes. Slice thinly against the grain.