In a large saucepan over medium-high heat, add the water and sugar. Bring to the boil, stirring constantly to dissolve the sugar.

Turn the heat down to medium and allow the mixture to come to a light amber colour without stirring. Instead, swirl the pan around occasionally to make sure the mixture caramelizes evenly. (This can take a while. Be patient, and don't be tempted to turn up the heat!)

Remove the pan from the heat, and add the pureed banana. Stir constantly until combined and the bubbling has stopped.

Place back on the heat, and keep stirring until the mixture becomes a deep, rich amber colour.

Remove from the heat and gradually pour in the heavy cream, stirring constantly until combined. Be careful, the caramel will bubble vigorously whilst adding the cream. Stir in the vanilla.

Pour the caramel into a heat-proof jar and allow to cool completely before using.