Toast the hazelnuts on a baking sheet, in a 350 degree F. oven for 10 minutes. Remove from the oven, and place in a kitchen towel. Rub the nuts inside the towel to remove their skins. Coarsely chop, and set aside.
Mix together the water, 1/2 cup sugar, and liqueur in a small pot. Bring to a boil, and stir until the sugar is dissolved. Remove from the heat and cool.
Line a deep bowl that is 9 inch in diameter, and 5 inches deep with plastic wrap. Cut the rectangular sponge cake into 9 squares, and then cut each square in half diagonally, creating 19 pieces of sponge cake in total. Brush both sides of the sponge cake with the liqueur mixture, and fit them together in the lined bowl so the mold is completely lined. If you have any spaces visibles, use spare pieces of the sponge cake moistened with the syrup to fill in the holes. Trim the top edge of the bowl evenly, and place it in the refrigerator while you prepare the filling.
Place the 2 cups of heavy cream with the icing sugar in a bowl, and beat with an electric mixer until the cream is in stiff peaks. Carefully fold in the hazelnuts, chopped chocolate and raspberries. Remove the mold from the refrigerator, and spoon the filling into the lined mold. Place the round cake on top of the filling, cover everything with plastic wrap, and refrigerate 12 hours.
Chop the chocolate finely, and place it in a bowl. Cook the cream until bubbly, and then pour it over the chocolate mixture. Let this mixture sit for 5 minutes, and then stir it until it is smooth. Leave sitting at room temperature. It will get thicker as it sits, but you need a mixture that is spreadable, but thick enough to nicely coat the bottom of the mold.
Remove the plastic wrap, place the cake on a wire rack top side down so the rounded end is on top, and remove the bowl. Place the wire rack over a baking sheet to catch the drips. Using a ladle, spoon the chocolate icing or ganache over the cake coating the entire surface. Decorate with extra hazelnuts and raspberries and serve.