To make the rub, stir together the cocoa powder, ancho chile powder, salt, black pepper, smoked paprika, garlic, cumin, and cinnamon. Generously rub onto both sides of the flank steak, and either let the steak rest uncovered at room temperature for 1 hour, or refrigerate covered overnight, then bring back to room temperature before cooking.
To cook the steak, heat the oil in a large cast-iron skillet on medium-high heat. After a couple of minutes, when the oil begins to shimmer, add the steak. Cook for 8 minutes, turning once, for medium-rare. There will be lots of sizzling and perhaps smoke, so you may want to open your windows. When the steak is done, remove it from the skillet and allow it to rest for 10 minutes before slicing it thinly against the grain.
As the steak cooks, warm the tortillas. Serve the steak with the tortillas, guacamole, and pico de gallo on the side at the table so people can make their own tacos.
Ancho Chile and cocoa-rubbed flank steak tacos | www.homesicktexan.com