Preheat the oven to 190°C/170°C Fan/gas mark 5/375ºF and butter a 1½ litre / 6 cups capacity pudding dish.Combine the 100g / ½ cup of dark muscovado sugar with the flour in a large bowl. Pour the milk into a measuring jug, beat in the egg, vanilla and melted butter and then pour this mixture over the sugar and flour, stirring — just with a wooden spoon — to combine. Fold in the dates then scrape into the prepared pudding dish. Don't worry if it doesn't look very full: it will do by the time it cooks.Sprinkle over the 200g / 1 cup of dark muscovado sugar and dot with the butter. Pour over 500ml/2 cups of boiling water (yes really!) and transfer to the oven. Set the timer for 45 minutes, though you might find the pudding needs 5 or 10 minutes more. The top of the pudding should be springy and spongy when it's cooked; underneath, the butter, dark muscovado sugar and boiling water will have turned into a rich, sticky sauce. Serve with vanilla ice cream, creme fraiche, double or single cream as you wish.