Butter a 1.5-litre (20cm) pudding basin. Add all the sauce ingredients to a saucepan and melt together slowly over a low heat, stirring continuously. Once melted, pour straight into the bottom of the basin and allow to cool slightly.
Cream the butter and sugar together in a stand mixer, or with an electric whisk, for 2 to 3 minutes, or until pale and fluffy. Add the remaining ingredients and whisk for a further 5 minutes until smooth and well combined.
Carefully spoon the mixture into the basin, over the cooled chocolate sauce, then gently spread out evenly so all the chocolate sauce is covered. Tear off a large piece of both kitchen foil and greaseproof paper (about an arm’s length) then lay both pieces over the pudding, greaseproof paper down, scrunching around the edges of the basin to seal tightly.
Place the pudding basin into a large, deep roasting tin and fill the pan halfway with boiling water. Place the pan over a high heat and bring to the boil, then cover with foil, turn the heat down to low and leave to steam for 2 hours, topping up with more boiling water if necessary.
Carefully remove the pudding from the pan and leave to stand for a minute or two. Carefully remove the kitchen foil and greaseproof paper and place a large serving plate over the pudding. Quickly flip over the basin and plate over, then lift off the basin to reveal the rich fudgy pudding.
Serve with your choice of custard, whipped vanilla-spiked cream or vanilla ice cream and dig in!
Grate a little extra chocolate over the pud to finish, if you like.