In a pan bring the rum, 250ml/9fl oz water and the vanilla seeds to the boil. Place the dates in a bowl, pour the liquid over the dates and leave to soak for an hour.
Preheat the oven to 180C/350F/Gas 4. Grease and flour 6-8 individual moulds or loaf tins (depending on how many people you are serving).
Cream the butter and the soft dark brown sugar with an electric hand-whisk in a bowl. Add the eggs one at a time, beating between each addition. Sift in the flour and bicarbonate of soda. Mix and then fold in the date and rum mixture.
Spoon the mixture into the prepared cake tins and bake in the oven for 12-15 minutes, or until just firm to the touch. Once cooked, remove from the oven and place on a wire rack to cool.
For the toffee sauce, put the sugar, syrup, treacle and butter into a pan and bring to the boil. Turn down the heat and cook, without stirring, until all the sugar crystals have dissolved and you have a deep-brown caramel. Meanwhile, gently warm the cream in a saucepan. Remove the caramel from the heat and carefully add one-third of the cream. Once the bubbling has subsided, whisk in the remaining cream. Add a pinch of salt and pass through a fine sieve. Set aside until needed.
Fan the banana slices into circles on top of the cooked puddings. Dust with demerara sugar and caramelise with a cooks’ blowtorch (alternatively place the puddings under a very hot grill). Set aside for a few minutes to allow the sugar to cool and set, then serve with the toffee sauce.