Preheat oven to 180 Celcius or 350 Fahrenheit. Set the baking ring to 22 - 24 cm (8.5 - 9") and line with baking paper. Do not grease the sides. Alternatively, bake in one deep 8" or to normal depth 8" cake pans.
Beat eggs for 3 minutes with a standmixer. Then add sugar and beat for another 12 minutes until the batter doubles in volume.
In a separate bowl stir together flour, ground hazelnuts, baking powder and cocoa powder.
Add dry ingredients to the wet and gently fold in with a spatula - gently fold in butter as well.
Pour the cake batter in the cake ring and bake in the oven for 30 minutes. Cool for 5 minutes in the cake pan, then remove from the pan.