Prepare the bananas. Peel the bananas and, using a sharp knife, carefully slice them in half lengthwise from top to bottom. This helps the bananas cook evenly. (Slice the bananas just before cooking, as they start to oxidize and brown quickly once cut. Freshly sliced bananas will give you the best texture)
Make the sauce. Melt the butter in a large skillet over medium-high heat, then mix in the brown sugar, vanilla, and cinnamon. Bring the mixture to a simmer.
Cook the bananas. Place the bananas cut side down in the skillet, and cook for 1 to 2 minutes, until softened and slightly caramelized. Carefully flip the bananas over. Keep an eye on the pan and gently shift the bananas occasionally to prevent them from sticking as they cook.
Flambé the sauce. Turn off the heat (this is important for safety) and add the dark rum to the skillet. Using a long-handled lighter, carefully ignite the sauce to flambé it. Gently shake or swirl the pan until the flames naturally subside.
Serve with Ice cream. Remove from the heat and transfer the bananas with sauce onto plates. Garnish with chopped nuts and top with a scoop of vanilla ice cream.