Get out a long lighter for igniting the rum later on.
In a skillet large enough to fit the bananas, melt the brown sugar and butter, stirring, until the butter has melted. Add in the banana liquer, then heat again until the mixture is bubbly.
Peel the bananas, then slice them lengthwise into two pieces. Cut each piece into half again, so that you get 4 pieces per banana.
Add the bananas to the brown sugar mixture in the skillet and cook until just heated, don't overcook.
Add the rum and heat up.
If desired, remove the pan from the heat and ignite the rum. Let it burn, shaking the pan to baste the bananas with the sauce and le the flames go out, then stir in the cinnamon and nutmeg.
Serve hot over the vanilla ice cream.
Notes
If you don't want to ignite the rum, you can simply add it and let it cook off in the sauce a bit, then add in the bananas for a final few minutes so they can heat up and soften.