Cut bananas in half so that both halves have all the skin on. Cut the halves lengthwise (so the bananas are shaped like boats) and LEAVE THE SKIN ON. Sprinkle with cinnamon sugar. (If using butter, brush bananas lightly before sprinkling with cinnamon sugar.)
Place bananas cut side down for 1 to 3 minutes or until bananas have grill marks. Turn over and continue cooking until warmed through. When the banana skin starts to pull away from the fruit, they're done.
Eat right out of the skin while still warm or serve on top of vanilla ice cream.