Preheat your barbecue grill to 350 to 400 degrees F. and spray the grill heavily with PAM or another grilling spray.
Combine the butter and brown sugar in a small saucepan and cook over medium-low heat until the butter is completely melted and the sugar is dissolved. With a sharp knife, cut each banana in half lengthwise, leaving the peel on the halves.
Brush the cut side of the banana halves with the sugar mixture and place the bananas cut side down on the grill. Grill until golden brown and caramelized, about 2 minutes. Re-spray the grill with PAM and turn the banana halves, peel side down and brush the upper surface of each with more glaze. Grill the bananas 2-3 minutes longer or until tender.
To serve, place two grilled banana halves in a sundae dish or shallow bowl. Top with two scoops of vanilla ice cream, drizzle with your favorite dessert sauce, and top with whipped cream and chopped nuts.
