Beat the sugar and egg yolks until frothy. Add the butter, lemon juice, cinnamon and salt and stir well to form a smooth mixture.
In a second bowl, stir together the flour, baking powder, hazelnuts and chocolate and slowly work into the foam (if the dough becomes too dry, 1 - 2 tablespoons of milk can be stirred in). Beat the snow out of the egg whites and carefully fold into the batter.
Pour into a greased and floured bundt cake pan and bake in a preheated oven at 190 ° C (top / bottom heat, hot air: 170 ° C) for about 45 minutes.