This steak is perfect for grilling or broiling. The combination of chili, cocoa, and brown sugar make for a steak that is smoky, sweet, bright, and subtly chocolaty all at once. You can serve it a la carte paired with a vinegary salsa, on tacos, or crunchy bread with cheese.
Season the flank steak evenly with kosher salt on both sides—about 1-2 1/2 teaspoons. Let sit for at least 30 minutes. Heat broiler to high and line a rimmed baking sheet with foil. Spray with nonstick spray and set aside.
In a small bowl, combine the brown sugar, cumin, coriander, chili powder, ground pepper, minced garlic, sweet paprika, and cocoa powder.
When the steak is room temperature, pat off any excess water with a paper towel, then season with the spice rub. Pour on the oil and rub the seasoning in so it adheres to the steak.
Transfer steak onto the baking sheet and place under the broiler on the top rack. Broil for 4 minutes, then flip over and broil for an additional 3-4 minutes for medium rare—the steak will be fragrant, caramelized, and lightly charred. Or, flash-sear it on the grill on high heat (one side only).
Remove from the oven (or grill) and allow to rest for at least 10 minutes before slicing thin and serving.