Preheat your sous vide machine to 148°F (64°C) and prepare the container.
Prep the chicken: place it breast-side down on a cutting board and use kitchen shears to cut along both sides of the backbone to remove it (spatchcock). Reserve or discard the backbone.
Press the chicken flat by pushing down on the breast so it lays evenly for cooking.
Season both sides of the flattened chicken with 1/2 teaspoon garlic salt and 1/4 teaspoon black pepper.
Place the seasoned chicken into a large sous vide bag, add 1 sprig of rosemary, arrange the chicken flat, and vacuum seal the bag.
Sous vide the sealed chicken at 148°F (64°C) for 6 hours.
When cooking is complete, remove the bag from the water and submerge the sealed bag in an ice bath for 5–10 minutes to cool; then discard the rosemary.
Preheat the oven broiler to High. Remove the chicken from the bag and pat it very dry with paper towels.
Place the chicken on a rack set over a lipped baking sheet, dot the top with 2 tablespoons butter, and broil on High for about 3–8 minutes until the skin is golden brown and crisp—watch closely to avoid burning.
Allow the chicken to rest for 15–30 minutes tented loosely with foil (leave a small vent) before carving into pieces to serve.