Mix the Čakovečki mlinovi flour, baking powder, salt and cocoa in a bowl and set aside. Melt the butter over low heat and let it cool a bit then mix it with sugar, vanilla sugar and eggs. Add yogurt and oil and mix briefly. Add the prepared dry ingredients slowly and mix the mixture with a mixer at lowest speed until all the lumps disappear.
Pour the batter into a floured kugelhopf tin. Bake for 50-55 minutes at 160°C. Use a toothpick to check if it is done.
Whisk the egg whites, and slowly add sugar, mixing constantly. Finally, add lemon juice and mix until the mixture is smooth and shiny. Carefully turn the cooled kugelhopf onto the serving plate and cover with icing.