Heat the oven to 180°C (160° fan).
Butter 4 ramekins.
Heat the milk in a pan to a boil.
Stir in the xylitol and polenta.
Remove from the heat and stir for about 3 minutes until thickened.
Slice the bananas.
Pour the polenta mixture into the ramekins.
Arrange the banana slices on top and press into the polenta.
Bake for about 15 minutes until set.
Sprinkle with powdered xylitol.