Preheat oven to 160 C. Put the cocoa powder, flour and baking powder in a separate bowl and mix them together.

Beat the eggs together with the brown sugar, vanilla sugar and salt until combined. Add the water, milk and sunflower oil. Mix everything together. Add the dry ingredients to the wet ingredients. Mix everything together into a smooth batter.

Pour the batter into the baking tin (don't forget the baking paper).

Place the chocolate cake in the preheated oven at 160 ℃ for about 30 to 35 minutes or until you pierce it with a skewer and come out dry.

Bring the water to the boil in a pan, together with the brown sugar and cocoa powder. Mix it with a whisk for a few minutes. When it boils well, turn off the heat. Let it cool down.

Pour the cold syrup over the warm cake and let it stand for an hour.

Put the whipped cream in a saucepan and bring it to a boil. Remove the saucepan from the heat and add the broken chocolate. Let it stand for a minute. After a minute, stir it together until smooth.

Divide the chocolate ganache over the cake, garnish with hazelnut pieces and cover. Now let the cake stand for a few hours, preferably overnight. This way the syrup absorbs well into the cake.