Beat the yolks with the sugar to a creamy mixture, add the yoghurt and stir. Sift the flour, vanilla, baking powder and cocoa. Add the egg mixture to the flour. Stir everything with a wooden spoon or spatula until you get a cake dough without floury lumps.
Put it in a slightly oiled tray and bake at 356°F (180 °C) for about 35-40 min. until your toothpick comes out dry when you poke it. Take it out and leave it to cool. Make the cream mixture by putting the sour cream and half the sugar in a bowl and stirring with a wooden spoon.
In another bowl, put the egg whites and beat until you get a foam, pour in the remaining sugar slowly until fully absorbed by the whites and they become snow-like.
Pour the resulting egg white mixture into the bowl with the cream mixture and stir carefully with a spoon.
Once the cake cools well, pour the entire mixture over it, smooth it out, sprinkle with grated chocolate and put it in the fridge. Leave the cake to cool for at least 12 hours, best if left overnight.