- In a small saucepan over high heat, combine the turkey brine and water and bring to a boil. Reduce the heat to medium-low and simmer, stirring often, until the brine dissolves, 5 to 10 minutes. Let the brine mixture cool to room temperature. In a large pot, stir together the brine mixture and buttermilk.
- Rinse the turkey inside and out with cold water and place in a large brining bag. Carefully pour the buttermilk brine mixture into the bag. Seal the bag, pressing out the air, and place in a large stockpot or other container large enough to hold the turkey. Refrigerate for 24 to 36 hours, turning occasionally.
- Remove the turkey from the brine; discard the brine. Rinse the turkey inside and out with cold water and pat dry with paper towels. Trim off and discard the excess fat. Place the turkey, breast side up, on a rack in a large roasting pan. Rub the skin evenly with the butter. Truss the turkey as desired using kitchen twine. Let the turkey stand at room temperature for 1 hour.
- Position a rack in the lower third of an oven and preheat to 400°F.
- Roast the turkey for 30 minutes. Reduce the oven temperature to 325ºF and continue roasting, basting every 30 minutes with the pan juices. If the breast begins to cook too quickly, tent it loosely with aluminum foil. After about 2 hours of total roasting time, begin testing for doneness by inserting an instant-read thermometer into the thickest part of the breast and thigh, away from the bone. The breast should register 165°F and the thigh, 175°F. Total roasting time should be 3 to 4 hours.
- Transfer the turkey to a carving board, cover loosely with foil and let rest for 20 to 30 minutes before carving. Serves 12 to 14.