Combine the water, sugar and butter in a large saucepa and bring to a boil. Cook for 1 minute then remove from heat and allow to cool down until just slightly warm.
In the meantime, sift the milk, cocoa powder and salt.
Pour the warm syrup over the dry ingredients. Add the vanilla and mix well.
Spoon the mixture into a 25x25cm square pan lined with parchment paper.
Top with roasted hazelnuts and place in the fridge to set for a few hours.
Cut into small bars to serve.
Notes
For the white version of this, replace the cocoa powder with powdered milk and add more vanilla to balance the taste out!