For best flavor, toast the hazelnuts in a dry skillet until the skins start to crackle before chopping.
This cake keeps well in the refrigerator for up to 5 days. Allow slices to come to room temperature for 10 minutes before serving for optimal texture.
You can substitute dark chocolate with milk chocolate for a sweeter, milder flavor profile.
If you prefer a crunchier texture, bake the nuts separately and add them after the cake has cooled, or reserve some to sprinkle on top just before serving.