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.