Toast cumin seeds and coriander seeds in a dry pan over medium heat until fragrant, about 3-4 minutes. Grind a spice grinder and then place in a medium bowl.
Add the instant coffee, brown sugar and all remaining spices (garlic powder through cayenne) to the bowl and mix well.
Rub the flank steak evenly on both sides with dry rub; set aside and let come to room temperature.
Heat grill over medium-high heat and grill steak for 7 minutes on each side for medium rare. Remove from heat and let rest 10 minutes before slicing.