Prepare the Spice Rub: In a bowl, mix chili powder, cumin, sugar, coriander, salt, pepper, cinnamon, and red pepper flakes. Rub the spice mixture over the flank steak, cover with plastic wrap, and refrigerate for at least 1 hour or up to 24 hours.
Make the Black Bean Salad: Preheat oven to 450°F (232°C). In a large bowl, whisk together 2 tablespoons oil, lime juice, scallions, and chipotle. Stir in black beans, bell pepper, and cilantro; set aside.
Cook the Steak: Pat steak dry with paper towels. Heat 1 tablespoon oil in the skillet over medium-high heat until just smoking. Place steak in the skillet and cook until well browned on one side (3–4 minutes). Flip steak, transfer skillet to the oven, and roast until the second side is browned and the internal temperature reaches 120–125°F for medium-rare (about 4–5 minutes).
Rest the Steak: Carefully remove the skillet from the oven, using potholders for safety. Transfer the steak to a cutting board, tent with foil, and let it rest while preparing the corn.
Toast the Corn and Serve: Wipe the skillet clean with paper towels and heat the remaining 1 teaspoon oil over medium heat until shimmering. Add corn and cook without stirring until well browned (5–7 minutes). Stir the toasted corn into the black bean salad and season with salt and pepper to taste. Slice the steak thinly against the grain and serve with the corn-bean salad.