Beat together bananas and butter until creamy. Add eggs, vanilla and water. Stir in flour, baking powder, bicarb, nutmeg and salt. Fold in prunes. Spoon mixture into a 23x33cm tin, or a 25cm Bundt tin, or two 23cm round cake tins.
Bake in the preheated oven until a knife inserted into the centre comes out clean. Time will vary depending on the tin used, but check after 20 to 25 minutes.