Beat the sugar with the butter (at room temperature) until you get a creamy mixture. Add the eggs one at a time, then mix in 200 g of ground hazelnuts, the espresso, cinnamon, and vanillin. Stir thoroughly.
Add the cocoa powder and flour previously mixed with the baking powder. Mix everything well.
Pour the batter into a greased cake pan and sprinkle the remaining ground hazelnuts on top. Bake at 180°C (350°F) for about 40–50 minutes.
Once baked, turn the cake out of the pan and, if desired, decorate with whole hazelnuts or walnuts.