Pre-heat the oven to 175°C. Grease a small springform (mine is 18cm or 6").
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.
Whisk the eggs and sugars together then pour in the chocolate mixture and stir until everything is combined.
Add the flour and cocoa and stir once more, just until everything is well incorporated. Pour into the prepared pan.
For a very runny kladdkaka (like my pictures), bake for 25 minutes. The cake is ready when the top is firm and the edges have started to pull away from the sides a little, but if you give it a tiny shake, the cake will still jiggle slightly. For a firmer middle, just extend the baking time to 30-35 minutes.
When ready, remove from the oven and allow to cool completely. When cooled, sprinkle with a very liberal amount of icing sugar.
For the Nutella cream: whip up the cream and fold in the Nutella. Leave in the fridge to set a little bit and then dollop on top of your kladdkaka when ready to serve.