Preheat your oven to 180°C/Fan 160°C/Gas 4. Use a little butter to grease a 900g loaf tin. Line the base with baking paper.
Blitz 75g of butter and 175g brown sugar together in a food processor. Add the eggs, treacle and flour along with a pinch of salt. Blitz again to form a thick cake batter, then scrape into a large bowl.
Put the dates into the food processor (no need to clean first) and add the bicarbonate of soda. Pour in 300ml of boiling water and blitz to a purée. Pour the date purée into the cake batter and whisk well to combine. Pour into your loaf tin and bake for 45-50 minutes until well-risen, and a skewer inserted into the centre comes out with a few dry crumbs attached. Leave to cool then remove from its tin.
Sauce time. Put the remaining 175g of butter, 175g of sugar, 2 tablespoons of miso, double cream and a pinch of salt into a saucepan. Cook over a medium heat, stirring occasionally, until the butter and sugar have melted and you are left with a glossy toffee sauce. Bring to the boil and bubble away for 10 minutes to thicken slightly. Taste, adding more miso if you like.
Slice the loaf into portions and transfer to a foil-lined baking tray. Spoon plenty of sauce over each slice, then cover with foil. Reheat in the oven for 10 minutes until warmed through. Transfer the warm pudding into bowls and serve with ice cream and more sauce.