1. In a medium bowl, add the brown sugar, onion powder, garlic powder, mustard powder, smoked paprika, pepper, tarragon, and salt and stir to combine. Use half of the spice rub to season the entire chicken. Cover with plastic wrap and refrigerate for at least 2 hours or overnight.
2. Preheat the grill to 190°C.
3. Remove the chicken from the fridge and season with the remaining spice rub. Pour out half the can of beer. Place the half-filled can in the chicken cavity and stand it upright.
4. Grill the chicken for 30-40 minutes. Baste the chicken with barbecue sauce and cook for another 10-15 minutes, until the internal temperature reaches 75°C.
5. Remove chicken from the grill and let rest for 15 minutes before carving.
6. Serve and enjoy!