Preheat your oven to 350 F / 180 C / Gas mark 4 . In a large mixing bowl beat the butter, sugar, and treacle together until the mixture is light and creamy in texture.
Slice open the vanilla pods and using a small sharp knife scrape out the seeds and add to the bowl. Keep the pods to make vanilla sugar. Sift the flour, baking powder and spices together and beat the eggs together in a small bowl.
Incorporate the egg a spoon at a time, beating into the creamed butter and sugar. If the mixture looks like splitting and curdling add a little flour.
When all of the egg is beaten into butter and sugar, fold in the flour using a metal spoon. Finally, mix in the mashed banana and date purée.
Butter and sprinkle on caster sugar to coat the bottom and sides of the baking tray or moulds. Spoon in the mixture trying not to dribble on the sides of the tins, then gently tap the bottoms on to the work surface to remove any air bubbles.
Place in the oven and cook for ten minutes then turn down the heat to 300 F /150 C / Gas mark 2 and continue to bake for one hour and twenty minutes. Cover with buttered greaseproof paper if necessary to prevent burning.
When a skewer can be inserted in the centre of the sponge and it comes out clean remove from the oven and leave to cool for ten minutes before turning out onto cooling racks.