Let the salmon sit at room temperature for 10 minutes while you preheat the oven to 375°F (190°C). Line a large baking dish or rimmed baking sheet with a piece of aluminum foil large enough to wrap the salmon and lightly coat the foil with nonstick spray.
Pat the salmon dry with paper towels and place it in the center of the prepared foil.
In a small bowl, combine the brown sugar (if using), chipotle chile powder, lime zest, and kosher salt.
Brush the salmon with olive oil (or melted butter) and the tablespoon of lime juice. Evenly sprinkle and gently rub the chipotle mixture over the top of the salmon.
Fold the foil up and over the salmon to form a sealed packet, leaving a little room inside for air circulation; use a second piece of foil if needed to fully enclose the fish.
Bake the salmon at 375°F for 12–18 minutes, depending on thickness, until it is almost cooked through at the thickest part (check earlier for ~1-inch thick pieces; thicker pieces may need longer).
Carefully open the foil to expose the top of the salmon (watch for steam). Switch the oven to broil and broil the salmon about 3 minutes until the top is slightly golden and the fish is cooked through, watching closely to avoid overcooking.
Remove from the oven; if still slightly underdone, reseal loosely and let rest a few minutes. The salmon is done when it flakes easily with a fork or reaches 145°F on an instant-read thermometer.
Cut into portions and sprinkle with chopped cilantro and an extra squeeze of lime juice if desired, then serve.
