In a large bowl, sift the flour, cocoa, baking powder and soda. Add the salt and sugar. Mix and set aside.<br></br>
Blitz the hazelnuts in a grinder or food processor until finely crumbed. Reserve 1/4 cup and add the remaining to the flour mixture. Stir to combine.<br></br>
Grease an 8" springform pan, at least 3 inches tall. Preheat the oven to 175 C.<br></br>
In a mixing bowl, combine the eggs, milk, oil and vanilla. Run a hand mixer through it to smoothen if needed.<br></br>
Fold in the flour mixture, alternating with the boiling water. You will have a thin batter and if there are any flour lumps, run the hand mixer through the batter again.<br></br>
Pour the batter into the prepared pan and bake for 30 minutes. Rotate the pan, and poke a toothpick in the center. The cake will likely still be very uncooked in the center, so bake for another 30 to 45 minutes. This will differ from oven to oven, so keep an eye on it. Cover with foil if the cake is darkening too quickly, though I found it rose quite tall so the foil stuck to the top a little.<br></br>
Allow the cake to cool completely. Run a knife around the edges, then open the pan. Level out the top of the cake with a sharp knife, then slice it horizontally in the middle.<br></br>
Slide a large metal spatula in the center and loosen the top layer. Place a plate upside down on it, and gently lift the top layer off and set aside. If you'd like to transfer the cake to a cake pan, loosen the bottom layer as well, then slowly slide it on the cake pan.<br></br>
To make the frosting, beat all the ingredients very well until light and fluffy. A hand mixer is best here.<br></br>
Mix the reserved ground hazelnuts into half the frosting. Spread this on the bottom layer of the cake.<br></br>
Now slowly flip the plate with the top layer on it, back on to the layer of frosting. Press down lightly.<br></br>
Top with the remaining frosting, add toasted hazelnuts on top. The cake is best eaten at room temperature, but if the frosting is very soft, chill it for 20 minutes. Happy baking!<br></br>