Put the dates in a heatproof bowl, cover with 150ml boiling water, and leave to soak for 30 mins. Butter a 1-litre pudding basin and line the base with baking parchment.
Tip half the butter, half the treacle, the vanilla, 75g of the sugar and the cream into a pan set over a medium heat. Cook for 4-5 mins, stirring, until the sugar dissolves. Turn up the heat, bubble for 3 mins, then whisk in a pinch of salt. Pour a third of the sauce into the basin.
Beat the remaining butter, treacle, sugar and the eggs together, then fold in the flour, bicarb, ¼ tsp salt, the dates and their soaking liquid. Spoon into the basin and smooth the surface, leaving a 1cm gap from the top. Cover with a double layer of baking parchment and foil, making a pleat in the middle so the pud can expand. Secure with kitchen string.
Set the slow cooker to low. Sit the basin inside, then add boiling water so it comes halfway up the basin. Cover and cook for 7-8 hrs. Run a knife around the edge of the pudding and turn out onto a plate. Reheat the remaining sauce and pour over. Serve with ice cream.