Place the chopped dates in a small bowl and pour over 175ml boiling water. Leave to soak for 15-20 minutes, until soft, then mash with a fork, retaining the water. Preheat the oven to 180°C/gas mark 4 and grease a two-litre baking dish.
Beat the softened butter together with the sugar until fluffy. Mix the eggs with the milk and vanilla in a small jug and gradually add to the butter mixture, beating well between each addition. Fold through the flour, bicarbonate of soda, mixed spice and baking powder. Add the mashed dates and stir through. Transfer to the dish and bake for 40-45 minutes, until risen and a skewer inserted in the middle comes out clean.
For the sauce, place the butter and sugars in a small saucepan over a low heat until melted, then turn up the heat and cook for 2 minutes. Add the double cream and cook for a further minute, until slightly thickened, then serve with the pudding.