In a bowl, mix well the Steak BBQ sauce, olive oil and parsley. Add the bavette steak and marinade to an airtight bag and refrigerate for 24 hours.
On a smoker-type BBQ, cook the flank steaks at a low temperature (225°F) for about 45 minutes or until the internal cooking of the meat is about 105°F.
Turn the temperature of the smoker up to maximum and allow a cast iron skillet to heat up for at least 15 minutes on high.
Stir the butter into the skillet and sear the bavette steaks well on both sides. Continue cooking until desired temperature is reached. For me, it is rare at 135°F of internal temperature. It can also be cooked to 145°F medium-rare, 165°F medium or 170°F well-cooked.
Allow to rest for at least 10 minutes in foil before cutting the meat.