The hot water and sugar mixture on top magically turns into a fudgy sauce in the oven - it will look like too much liquid but trust us, it works! We've served this with ice cream, chopped hazelnuts, and espresso powder, but you can serve with whatever you like!
Preheat oven to 180 C. Butter a large cast iron skillet.
Whisk together Nutella, egg, milk, and melted butter until smooth. The Nutella may make it a little bitty at the start, but just keep whisking and it'll come together.
In a separate bowl, combine all the dry ingredients. Add the wet ingredients, and mix until incorporated. Pour into the skillet and spread out.
Mix together the cocoa and brown sugar. Sprinkle over the top of the skillet.
Whisk together the boiling water and coffee liquor. Over the back of a large spoon, carefully pour the mixture over the cocoa and sugar. Don't mix! Carefully pop the whole thing into the oven for 35-40 minutes, or until it springs back to the touch.
Top with espresso powder, ice cream, and chopped hazelnuts. Dig in!