1) Mix together the chili powder, instant coffee, cumin, and brown sugar in a small bowl until thoroughly combined, using a fork or whisk.
2) Put flank steak on plate and sprinkle with salt and pepper on both sides, then take the rub mixture and gently press into both sides of the flank steak.
3) Spray broiler pan with cooking spray and place steak on it.
4) Turn broiler on and allow to heat. Slide broiler pan under broiler so steak is 4 inches from heat source. Cook steak for 12 minutes or until done to your liking. DO NOT FLIP STEAK.
5) Remove broiler pan from oven and cover steak loosely with a piece of tin foil. Let stand for 5 minutes before cutting.
6) To cut, slice diagonally, against the grain, in thin slices.