Melt the butter in a skillet over medium heat. Stir in the sugar, rum, vanilla and cinnamon. Whisk until sugar is fully dissolved. Light the skillet with a flame to flambe the sauce (optional).
Add 8 slices of banana to the skillet. Allow the sauce to bubble for about 5 minutes until caramelized. Remove from heat and cool. Extract the banana slices from the sauce. These will be used for the garnish, so thread them onto a toothpick and set aside.
To make the cocktail, mix the rum, 1/2 the bananas foster sauce, the remaining banana and ice to a blender. Blend until smooth.
Prepare the glasses by pouring the remaining banana fosters sauce onto the inside wall of the glasses. Pour in the blended cocktail. Garnish with the skewer of bananas.