Separate the Egg Yolks and Egg Whites in two different bowls. Set aside.
Using a stand mixer on medium-low speed for about 2 minutes, blend the Butter, Sugar, Vanilla Sugar, and pinch of Salt into a smooth texture. Reduce speed to low and add in the Egg Yolks one at a time. Add the Lemon Zest and Lemon Juice. Mix well.
Add the Flour and mix well until it forms a smooth batter. Remove bowl from stand mixer and quickly knead the dough in the mixing bowl for 1-2 minutes.
Place the dough onto a floured work space and shape it into a thick rectangle. Cut the rectangle in half. Cut one of the halves into two smaller halves. Wrap the pieces separately in plastic wrap. Refrigerate the large piece and freeze the two smaller pieces for 1 hour.
Prepare the filling. In a stand mixer, add Cream of Tartar into the Egg Whites and mix on low-medium speed until light and foamy. Gradually add in the Granulated Sugar and increase the mixer speed to high. Mix until you get nice stiff peaks. Do not overmix. Set aside.
In a new mixing bowl, use a spatula to gently mix the Yogurt together with the Sour Cream. Add the Vanilla Sugar, mix well. Add the Flour and mix well again.
Gently fold 1/3 of the Egg White mixture into the Yogurt mixture. Repeat the same procedure twice. Change to a Whisk. Whisk the mixture till well combined.
Take the large piece of dough from the fridge. Roll and stretch it to fit the bottom (not the sides) of a greased 20 x 27 cm oven pan.
Pour the Yogurt mixture into the pan and spread it evenly.
Use a grater to grate the frozen dough loosely on top of the filling.
Bake the Yogurt Cake in a preheated oven at 180 C (350 F) for 20 minutes, then turn the heat down to 150 C (300 F) and bake for another 20 minutes. Turn off the heat, slightly open the oven door, and leave the cake in the oven for 45 to 60 minutes. Before serving, dust it with Powdered Sugar!
When preparing the dough, you can also wrap it in a plastic wrap to help you shape the dough as shown in my video.
While I prefer to eat this Dreamy Lemon Yogurt Cake when it is freshly out of the oven, it can be easily stored and covered in the fridge for a few days.
When placing the dough in the bottom of your pan, use your finger tips to push the dough evenly into place.