For the bananas Foster: Add the butter and the bananas to a medium nonstick skillet over medium heat and cook until the bananas are caramelized, about 2 minutes per side. Stir in the brown sugar and cinnamon. Pulling the saute pan away from the flame, add the banana liqueur and stir to combine. Stir in the vanilla extract and salt, and scrape the bananas Foster into a bowl to cool.
For the milkshake: Combine the rum, the cooled bananas Foster and the ice cream in a blender and blend until smooth and creamy. Pour into glasses and top with a dollop of the whipped cream. Dust with cinnamon, garnish with a maraschino cherry, and serve with a long straw.