Preheat the oven to 180C/160C Fan/Gas 4. Butter an ovenproof dish – either a large traditional oval one or a brownie tin measuring about 20 x 30cm.
Put the dates and bicarb in a bowl and cover with the just-boiled water then leave to stand
Put the 50g of butter and the sugars in a bowl and beat with an electric beater until very soft and fluffy. Add the eggs, one a time, adding a heaped tablespoon of flour with each addition. Fold in the rest of the flour.
Add the dates and their soaking water and stir briefly to combine, then pour into the prepared dish. Bake the pudding in the oven for 25-30 minutes, or until it is springy to the touch and slightly shrinking away from the sides.
While the pudding is baking, make the sauce. Put all the ingredients in a pan, place over a low heat and stir until the butter has melted and the sugars have dissolved. Turn up the heat and bring to the boil, then simmer for a few minutes, until the sauce has thickened to the consistency of a light custard - it should coat the back of a spoon.
While the pudding is still hot, poke holes over the surface and pour over half the sauce. Leave the pudding to stand for a while – if you like, you can leave it for a couple of days and it will just get stickier.
When you’re ready to eat, warm it through again and serve with the rest of the sauce, also warmed through. Serve with cream or ice cream.