Begin by removing the crusts from the slices of bread, which should leave you with 9 pieces about 4 inches (10 cm) square.
So now cut each slice into 4 triangles. Next, place the chocolate, whipping cream, rum, sugar, butter and cinnamon in a bowl set over a saucepan of barely simmering water, being careful not to let the bowl touch the water, then wait until the butter and chocolate have melted and the sugar has completely dissolved. Next, remove the bowl from the heat and give it a really good stir to amalgamate all the ingredients.
Now in a separate bowl, whisk the eggs and then pour the chocolate mixture over them and whisk again very thoroughly to blend them together. Then spoon about a ½ inch (1 cm) layer of the chocolate mixture into the base of the dish and arrange half the bread triangles over the chocolate in overlapping rows. Now, pour half the remaining chocolate mixture all over the bread as evenly as possible, then arrange the rest of the triangles over that, finishing off with a layer of chocolate. Use a fork to press the bread gently down so that it gets covered very evenly with the liquid as it cools.
Cover the dish with clingfilm and allow to stand at room temperature for 2 hours before transferring it to the fridge for a minimum of 24 (but preferably 48) hours before cooking.
When you’re ready to cook the pudding, pre-heat the oven to gas mark 4, 350°F (180°C). Remove the clingfilm and bake in the oven on a high shelf for 30-35 minutes, by which time the top will be crunchy and the inside soft and squidgy. Leave it to stand for 10 minutes before serving with well-chilled whipped cream poured over.