Poke the sweet potatoes with a fork all over, about six times. Place them on a baking sheet lined with foil and bake until very tender, about 45 minutes to 1 hour.
Cut the sweet potatoes in half lengthwise and, with a knife, make crosshatch marks, cutting into the potatoes about halfway through. Place the sweet potatoes on the baking sheet and set aside.
In a medium bowl, combine the brown sugar, butter, and pecans (if using). Spoon the mixture onto the 8 sweet-potato halves. Heat in oven until the potatoes are hot and the sugar and nuts are browned, about 20 to 25 minutes. Serve hot or warm.