Preheat oven to 350 degrees. Place hazelnuts on a baking sheet; toast until fragrant, about 10 minutes. Transfer nuts to a kitchen towel; rub off loosened papery skins. Let stand until cool. In the bowl of a food processor fitted with the steel blade, pulse nuts until they are the texture of coarse cornmeal; set aside.⏱ 10 min
Place a layer of chocolate dacquoise on a 10-inch cardboard cake round, and coat with a 1/2-inch-thick layer (1 cup) ganache. Top with second layer and spread 1 1/2 cups buttercream in an even layer over dacquoise. Top with third layer and coat with the remaining cup of ganache. Top with final layer, placing it upside-down so the flat side is on top, and coat with about 2 cups buttercream. Refrigerate for 20 minutes.⏱ 20 min
Coat the top and sides of cake with about 3 cups buttercream in an even layer. Cover top and sides of cake with hazelnuts. Use remaining buttercream to decorate, as desired. Reserve any remaining buttercream for another use.