Preheat the oven to 200 degrees Celsius/ 400 degrees Fahrenheit. Line a springform 24 cm diameter/ 9.8 inches with baking paper, the ring included.
Beat eggs: Place the eggs and the sugar in the bowl of the food processor and beat until pale and fluffy, about 10 minutes.
Roast hazelnuts: In the meantime, roast all the ground hazelnuts, included those needed for the filling, in a non-stick pan without any fat. Stand by and shake the pan often. When roasted, transfer immediately to a plate and let cool.
Melt the butter and set aside until needed.
Mix the flour, corn starch, and baking powder in a small bowl and sieve over the egg mixture. Fold in gently.
Add the butter and half of the roasted hazelnuts and fold in as well. Keep the rest of the hazelnuts for the filling.
Bake: Pour the mixture into the prepared form and bake for about 30 minutes or until a skewer inserted in the middle comes out clean. Let the cake cool in the pan for about 10 minutes, take out of the form, and place on a wire rack to cool completely.
Cool: Once cool, wrap in aluminum foil if you let it rest until the next day. This will prevent it from drying.