For the hazelnuts: Pre-heat the oven to 200ºC (400ºF) and roast the hazelnuts for around six minutes on a large baking sheet. Remove from the oven and allow to cool for several minutes. Try to loosen as many skins as you can and discard them (don't worry if you don't get them all).
Heat the remaining ingredients for the caramelised hazelnuts in a pan until the mixture thickens (around 4-5 minutes). Add the hazelnuts, stir to combine and then spread them out on a tray lined with baking paper. Allow them to set and then crush them into small pieces in a food processor.
Add the baking powder, flour and cocoa and mix until everything comes together. Finally, stir in the chopped chocolate and hazelnuts (retaining around 2 tablespoons to sprinkle later). Spoon the batter into the skillet and evenly spread it out. Bake for around 20-25 minutes (if you would like a soft centre, reduce this time).
Remove from the oven and wait around five minutes before melting the dark chocolate and drizzling it over along with the remaining crushed hazelnuts. Slice and serve with a dollop of ice-cream on top. Enjoy!