Preheat the oven to 165 - 175 degrees top / bottom heat.
Mix the butter with the hand mixer until foamy and gradually stir in a little sugar and 1 egg. Stir in each egg for about 1 minute. Then stir in the flour mixed with baking powder, hazelnuts, bitter almond oil and the chopped chocolate.
Put the dough in a greased wreath form dusted with flour and bake on the middle rack for 50 - 65 minutes. The baking time varies depending on the oven and the size of the baking pan (mine is 26 cm in diameter). So it`s better to do a chopstick test and check the cake after 50 minutes!
When the cake has cooled down, either dust it with powdered sugar or cover it with melted couverture.