Combine beef stock, soy sauce, and chipotle peppers in a blender; blend until smooth. Pour into a resealable plastic bag and add flank steak. Close bag and move steak around so it is well covered in marinade. Refrigerate for at least 4 hours, or overnight.
Remove bag from the refrigerator 20 minutes before grilling. Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Remove steak from the marinade and shake off excess liquid. Cook steak until firm and reddish-pink in the center, 4 to 5 minutes minutes per side, depending on the thickness. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C) for medium-rare. Let rest for a few minutes before slicing.
