Combine marinade ingredients in a zip top bag or glass casserole dish.
Add flank steak and make sure it is coated with marinade. Refrigerate for a minimum of 2 hours or overnight, turning once or twice.
Prepare grill for direct high heat (400°F). Remove flank steak from marinade and place on the grill, reserving marinade for basting.
Grill for 3 minutes, flip steak over, and brush with marinade. Grill for 3 more minutes, flip and brush with marinade again. Check the temperature with an instant-read thermometer. Grill for about 1 minute on each side for rare (take off at 125°F), or 2-3 more minutes per side for medium rare (take off at 130°F). Be sure to check the temperature a few minutes before you think it will be ready.
Remove steak from the grill and place on a cutting board. Tent with foil and allow to rest for 5 or more minutes. Temperature of the steak will increase while resting.
Slice the flank steak thinly across the grain and diagonally (at a 45 degree angle from top to bottom).
Ancho chili powder packs a bit of a punch, so use more, or less, depending on preference. Paprika can be substituted in its place if desired.
Be sure to remove steak from grill 5 degrees before desired doneness as the temperature will rise while it rests.