1. Preheat the oven to 200 °C (400 °F). Bring the milk to a boil in a saucepan with the split vanilla bean. Let it infuse for 10 minutes. Remove the vanilla bean.
2. Pour the milk over the bread. Mash with a fork. Peel the bananas and slice them. Reserve a few slices for decoration.
3. In a blender, combine the bananas, milk-soaked bread, egg yolks, sweetener, cinnamon, and margarine. Blend until smooth.
4. Pour the mixture into a non-stick mold, arrange the remaining banana slices on top, and bake for 30 to 40 minutes.