Place the dates in a bowl and pour over 200ml/7fl oz boiling water; leave to soak for about 20-30 minutes. Meanwhile, preheat the oven to 180C/350F/Gas 4. Lightly butter a 1 litre/2 pint ovenproof dish.
Stir the butter, sugar, eggs, vanilla and walnuts into the bowl containing the dates and their soaking liquid. Mix until smooth, then sift the flour and baking powder and fold in gently until the mixture is evenly blended.
Bake for 40-45 minutes until the pudding is firm to the touch. Make the sauce towards the end of the pudding's cooking time: measure the sugar, butter and five tablespoons of cold water into a small pan. Heat gently, stirring, until the butter has melted, then allow to simmer for 2-3 minutes to thicken the sauce.
Remove the pudding from the oven and pour the sauce evenly over the top. Serve hot on its own or with single cream.