To make the sauce, grease a 2-pint pudding basin with a little butter and set aside. Place a large heatproof mixing bowl over a pan of simmering water (make sure it’s not too tight a fit or the steam will burn you). Add all of your sauce ingredients and allow to melt, stirring frequently until blended and glossy. Remove from the heat.
Pour about 8 tablespoons of your lovely sauce into the bottom of the greased pudding basin and put the rest to one side for later.
Next, pop all the pudding ingredients into a food processor and blitz until nice and smooth. You can also do this by hand with a wooden spoon if you don’t have a food processor. Gently transfer the mixture to the pudding basin, on top of the sauce.
Cover the pudding basin with some greaseproof paper and a layer of foil to hold it in place. Scrunch this around the edges and tie some string around the basin to secure it. At the same time, make a handle from the string so it is easier to handle the pudding basin later on.
When you’re ready to cook your pudding, get a saucepan large enough to hold the pudding basin and have its lid on tightly, so try it for size first. Once you’ve got a good fit, place a heatproof plate on the bottom of the pan - this will stop the bottom of the pudding from burning.
Fill the pan with a couple of inches of boiling water from the kettle and carefully lower your pudding in. The water should come halfway up your pudding, so top it up if you need to.
Bring the water to the boil, put the lid on, then simmer for about 1½ hours, making sure you keep the water topped up when needed so it doesn’t boil dry. After this time, carefully remove your pudding from the pan.
Turn the pudding out onto a nice big platter, but keep in mind that the chocolate sauce is going to be hot and runny, so be very careful. Place the pudding in the centre of your table with a jug of warmed custard and a jug containing the remaining warmed chocolate sauce. Top the pudding with a little holly and let everyone tuck in!