In a large bowl, sift together the flour, baking powder and cocoa powder. Add the ground hazelnuts and mix to combine. Set aside.
In a stand mixer with paddle attachment or large bowl with hand mixer, beat the butter and then add the sugar and vanilla extract. Beat until creamy. Add the egg and beat again. Slowly add the dry ingredients and mix until dough starts to come together. Use your hands to finish mixing the dough as it will be stiff like cookie dough. Divide the dough into four equal pieces and chill for 30 minutes (up to 24 hours) in the fridge.
Preheat the oven to 375 degrees F. Remove the sides of a 26cm spring form pan and grease the bottom with butter.
Lightly flour a pastry mat, remove one piece of dough from the fridge at a time, flatten it into a disk and roll it out evenly until it is slightly larger than the spring form bottom. Gently fold the dough in half, place it onto the spring form pan bottom and unfold the dough. Press any cracks together and cut off excess dough to create a flat, vertical edge all the way around.
Bake without the spring form ring around it for 10 minutes or until golden brown. Transfer to a wire rack to cool and repeat steps above with the other three pieces of dough. When the last piece comes out of the oven, cut it into 16 equal slices, let cool and then gently transfer each piece to a wire rack.
Using a double boiler or small pot with heat proof bowl on top, melt and combine the chocolate and coconut oil. Coat the 16 slices evenly on one side using a spoon to drizzle the chocolate on each piece and a pastry brush or knife to spread the chocolate to the edges. Set aside.
In a small saucepan mix the gelatine with the water and let it sit for a few minutes to swell until all the water has been absorbed. Add the lemon zest and sugar and mix to combine, then warm over low heat until the sugar and gelatine melt. Remove from the heat and add lemon juice. Stir to combine then set aside to cool.
In a stand mixer with whisk attachment or large bowl with hand mixer, whip the cream until almost stiff peaks, then add the sifted icing sugar, turn mixer on low and add the gelatine mixture. Whip again to combine and until stiff peaks form.
Fill some of the cream into a piping bag with a 1M sized tip. Place one cake layer on a wire rack and use a small, sharp knife to gently etch 16 slices on it as guidelines. Pipe the cream onto each segment in a cone-shaped pattern, starting in the center and increasing the size of the cone towards the outside edge. Set aside.
Place the bottom layer of cake on a flat cake plate or stand and pipe cream around the edge and into the center to cover the entire layer of cake. Place another layer of cake on top and repeat piping around the edge into the center. Save a little bit of cream for the decoration in the center of the top layer. Place the layer with the piped cone-shaped patterns on top. Take each chocolate slice and place it on its side, leaning against a cone, with the point towards the middle. Pipe a swirl in the very middle where all the slices meet. Sprinkle with chocolate shavings and press lemon wedge into swirl.
Chill for at least 4 hours before serving to allow the whipping cream to soften the cake layers.
1. If you have more than one spring form pan of the same size, you can bake two layers of cake at the same time.
2. If the decoration above seems too daunting you can assemble it how my Mom and I used to. Don't cut the last layer into 16 slices or top with chocolate icing. Stack the cake layers and pipe or spread the lemon whipped cream evenly between all four layers of cake. Pour all the chocolate icing on top of the last layer and spread it evenly to the edges, allowing some of the chocolate icing to drip down the sides of the cake. Top with a small pile of curled lemon zest before serving.