In a small bowl, mix the ground coffee, brown sugar, smoked paprika, garlic powder, onion powder, cumin, cayenne pepper, salt, and black pepper to create the coffee rub.
Pat the flank steak dry with paper towels and rub the coffee blend evenly over the meat, pressing it into the surface.
Let the steak sit at room temperature for about 30 minutes to allow the flavors to absorb.
Preheat your grill or skillet to high heat. Cook the steak for about 5-7 minutes per side for medium-rare, or until your desired doneness is reached.
Remove the steak from the heat and let it rest for about 5 minutes before slicing it thinly against the grain.
While the steak is resting, warm the corn tortillas on the grill or in a dry skillet for about 30 seconds on each side until they are pliable.
To assemble the tacos, place a few slices of steak on each tortilla, then top with diced tomatoes, avocado slices, and chopped cilantro.
Serve with lime wedges on the side to squeeze over the tacos for added freshness.
Notes
Let the steak rest after cooking for better flavor.