Place Hazelnuts in a large sauté pan over medium heat, stir until toasted, or, place on a cookie sheet then place in hot oven for about 20-minutes. Remove and place in between a kitchen towel, allowing the steam to loosen the skins. Place an oven mitt over the towel and rub robustly to break off the skins. Separate the Hazelnuts from the skins. [Alternatively, you may purchase blanched Hazelnuts.]
Place the Hazelnuts (without the skins) in your food processor and run until the hazelnuts have become a smooth buttery paste.
Add hot water to the bottom portion of your double-boiler, then place in the top portion of the boiler the two Chocolates, Sugar, Milk, and Butter, and bring to a boil over medium-high heat; cook stirring often, until the Sugar dissolves, Chocolate melts, and the mixture is smooth and shiny.
Then, add the Hazelnut paste and continue to stir until smooth again.
Remove from heat and stir in the Vanilla.
Let icing sit until thick enough to spread, about 30-minutes.