Special equipment: 6 to 8 individual pudding molds or loaf tins and a kitchen blowtorch
For the pudding: In a pan, bring the rum, vanilla seeds and 1 cup plus 2 tablespoons water to the boil. Place the dates in a bowl, pour the liquid over the dates and leave to soak for 1 hour.
Preheat the oven to 350 degrees F. Grease and flour 6 to 8 individual pudding molds or loaf tins (depending on how many people you are serving).
Cream the butter and dark brown sugar with an electric hand mixer in a bowl. Add the eggs one at a time, beating between each addition. Sift in the flour and baking soda. Mix, and then fold in the date and rum mixture.
Spoon the mixture into the prepared molds and bake until just firm to the touch, 12 to 15 minutes. Place on a wire rack to cool.
For the toffee sauce: Put the sugar, butter, syrup and treacle 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 caramelize with a kitchen blowtorch (alternatively place the puddings under the broiler). Set aside for a few minutes to allow the sugar to cool and set, then serve with the toffee sauce.
Double cream, which is similar to heavy cream and has a butterfat content of 48 percent, can be found in specialty stores.
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.