Grease a 1 litre pudding basin with butter and coat with demerara sugar.
Place a heatproof plate in a large saucepan and boil a kettle of water.
Cook dates in 200ml of water for 4 minutes then blend with vanilla paste.
Beat 50g butter and brown sugar until smooth.
Mix in syrup, treacle, flour, eggs, bicarbonate of soda, and date purée.
Pour batter into the basin and seal with pleated greased foil and string.
Steam the basin in the saucepan with 50% submersion in boiling water for 3 hours.
Melt cream, 175g butter, and sugar in a pan then whisk in syrup and treacle.
Remove the pudding from the pan and turn out onto a plate.
Serve slices of pudding with warm sauce and vanilla ice cream.