In a bowl, whisk together cocoa powder, chili powder, chipotle chili powder, garlic powder, onion powder, brown sugar, kosher salt, black pepper, cumin, and smoked paprika.
Lightly drizzle the flank steak with olive oil and rub the cocoa rub on both sides.
Heat your grill to 500˚F.
Grill the flank steak for 3 to 5 minutes per side, or until the internal temperature reads 135ºF.
Remove the flank steak, and let rest for 15 minutes.
Slice against the grain on a sharp diagonal and serve.