Line the slow cooker insert with a single layer of heavy-duty aluminum foil, shiny side up, to catch drips. Follow your model’s instructions for foil, minimum fill and liquid requirements.

Pat the chicken breasts dry with paper towels and trim excess fat. If a breast is especially thick, you may slice it in half horizontally for more even cooking; keep the chicken divided among four portions.

Combine the packed light brown sugar and dry Italian dressing mix in a small bowl. Stir with a fork until the seasoning is evenly distributed and there are no large sugar clumps.

Tear off four large sheets of heavy-duty foil. Lay them shiny side up on the counter, leaving enough foil around each portion to fold and seal securely.

Divide the chicken among the four foil sheets. Spoon the brown sugar mixture evenly over the portions, pressing it lightly onto the tops and letting some fall around the sides. For the optional butter version, place one pat of unsalted butter on each portion before sealing.

Bring the long sides of each foil sheet together over the chicken and fold them down. Roll and crimp the ends to close each packet securely, keeping the seams at the top.

Arrange the packets in the slow cooker in a single layer as much as possible, overlapping slightly only if needed. They should fit without being crushed, and the lid must close properly.

Cover and cook on LOW for 4 to 5 hours or HIGH for 2 to 3 hours, until the chicken is cooked through and a food thermometer reads at least 165°F (74°C) in the thickest part of each portion. Thickness and cooker performance affect timing, so check the temperature rather than relying on the clock alone.

Turn off the cooker and let the packets rest inside for about 5 minutes. Lift them onto plates with tongs, supporting the bottoms. Open them away from your face to release the hot steam, then spoon the juices over the chicken before serving.

Let the insert cool before removing the foil lining and used packets. Wash the insert according to the manufacturer’s directions before putting it away, even if the foil caught most of the juices.