Separate the eggs. Whisk the egg whites together with the salt in a food processor to form a snow. Pour into a bowl and set aside.
Preheat the oven to 170°C. Prepare a cake tin (approx. Ø 25 cm). If it is not coated, grease it slightly.
Beat the softened butter and sugar for a few minutes until fluffy. Gradually add a yolk and mix for another 2 minutes.
Mix hazelnuts, corn starch, baking powder and cinnamon in a bowl. Fold into the yolk-butter-sugar mixture until combined. Fold in the beaten egg white with a whisk.
Empty the dough into the cake tin and bake for 35-40 minutes. No dough should stick if you put a skewer in it.
Let the cake cool down in the mould for a few minutes. Then turn out onto a plate and let it cool completely.
Place the chocolate and coconut oil in a heat-resistant container and melt over a water bath. Stir until the chocolate is smooth. Glaze the cake with it. Refrigerate in the fridge. Before serving, garnish the chocolate cake with fresh berries.