How to Make It
Preheat the oven to 350°. In a medium ovenproof skillet, melt 1 tablespoon of the butter. Add the hazelnuts and cook over moderate heat until just beginning to brown in spots, about 2 minutes. Transfer the skillet to the oven and roast the nuts for about 6 minutes, until the skins blister. Transfer the nuts to a kitchen towel and rub off the skins. Let cool.
In a food processor, combine the hazelnuts with both chocolates and process until the nuts are finely chopped and the chocolate just begins to melt, 4 minutes.
In a small saucepan, heat the sweetened condensed milk with the salt until hot. With the food processor on, add the hot condensed milk and process until incorporated. Add the remaining 7 tablespoons of butter and process until smooth, 2 minutes. Add the boiling water and process until creamy, 2 minutes.
The spread can be refrigerated for up to 3 weeks. Bring to room temperature and add a few drops of boiling water to smooth the texture.
Toast, bananas or apples.