Pre-heat the oven to 175°C / 350℉. Prepare a muffin tin (see note
Add the butter, Nutella and chocolate to a pan and very slowly heat until just melted.
Add the coffee (or whatever you are using) and stir to combine.
Take off the heat as soon as you've stirred in the liquid. It shouldn't be hot, but if it is, just let it sit for a while to cool down.
Whisk the eggs and sugars together then pour in the chocolate mixture and quickly whisk until everything is combined.
Add the flour and cocoa and whisk once more, just until everything is well incorporated.
Pour, or spoon, the batter into the cases (it will be quite runny) about half way up (see note 4).
Spoon one teaspoon of Nutella into each muffin case (you can use a little more if you like Nutella).
Cover with the remaining batter (your cases should be about ¾ full, but it doesn't matter if it is a little more as these cakes do not rise a great deal).
Sprinkle over a generous amount of chocolate chunks. Place in the oven (see note 5).
When ready, remove from the oven and allow to cool completely. Don't panic when they implode - that means you've done something right! Enjoy (with or without a spoon).