Pre-heat the oven to 175°C (350℉). Grease a small springform (mine is 18cm or 7").
Add the butter, Nutella and chocolate to a pan and slowly heat until just melted.
Add the coffee (or water) 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.
In a bowl, whisk the eggs and sugars together then pour in the chocolate mixture and stir until everything is combined.
Sift the flour and cocoa together and stir in, just until everything is incorporated. Pour into the prepared pan.
See note 3 for baking instructions.
When ready, remove from the oven and allow to cool completely in the pan. As the cake will be very soft (even runny), you can't remove it from the pan as you would a regular cake.
When cooled, sprinkle with a very liberal amount of icing sugar.