Whisk all ingredients except the butter till thoroughly combined until smooth
Brush each steak with 1 tablespoon of the mole sauce and let the steaks stand at room temperature for 1 hour.
Light a grill. Season the steaks on both sides with salt and pepper. Grill them over high heat for 3 to 4 minutes per side for rare to medium-rare meat. Transfer to a platter and let rest for 5 minutes.
Meanwhile, bring the remaining mole sauce to a simmer. Whisk in the butter, 1 tablespoon at a time.