For cake: Preheat oven to 350 degrees F. Place all hazelnuts, including those for the topping, on a jellyroll pan and place in oven for 15 minutes. Remove pan (leave oven on) and cover with a clean kitchen towel for 10 minutes. Remove towel and grab a small handful of nuts and rub them between your hands, loosening skins. (Not all skins will come off). Separate nuts into 1-1/2 and 1 cup portions.
Place 1-1/2 cups of nuts in a grinder, a little at a time, and grind until pieces are fairly small but not into powder.
In a large bowl, beat butter and half of the sugar until soft and light. Beat in melted chocolate and then egg yolks, one at a time. Add ground hazelnuts, flour and cinnamon, stirring to blend.
In a separate bowl, beat egg whites until they hold a solf peak. Slowly beat in remaining sugar until mixture holds firm peaks.
Stir in about 1/4 of the egg whites to the batter. Add remaining egg whites and gently fold in until completely blended.
Pour batter into jellyroll pan and bake for about 25 to 35 minutes or until firm in center. Remove from oven and turn cake over onto a rack and let sit for a few minutes. Remove pan and paper and let cool completely.
For filling: In a small saucepan bring cream, butter and corn syrup to a boil. Remove mixture from heat and add chocolate, submerging it in the cream mixture. Whisk until all the chocolate has melted and mixture is smooth. Transfer to a bowl and let cool.
To finish: Chop remaining 1 cup of nuts. Cut cake into two layers. Place one layer on a round of cardboard or a plate and spread with half the filling. Add second layer and frost sides and top of cake. Grab a handful of nuts and press them into sides and top of cake. Serve immediately.
For gf flour: 2 cups rice flour 2/3 cup potato starch 1/3 cup corn starch 1/4 teaspoon xantham gum Mix all ingredients together until well blended.