Mix the flour with the baking powder and powdered sugar, Crumble the cold butter into it.

Add the egg yolks, vanilla sugar and grated lemon zest.

Mix in enough sour cream to make a stretchable, elastic dough.

Cover the finished dough and let it rest in the refrigerator for at least an hour.

In the meantime, Prepare the filling.

Beat the egg whites with the powdered sugar until stiff.

Carefully fold in the ground walnuts, lemon zest, and cinnamon to create a light, airy mixture.

Divide the rested dough into two parts, two-thirds and one-third.

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Roll out the larger part on a floured surface and place it in a 25 × 35 cm baking pan lined with baking paper.

Spread the top of the dough evenly with apricot jam. I then place the walnut mousse on top and smooth it out.

Roll out the reserved dough, cut it into strips, and place them on top of the filling in a lattice pattern.

Brush the lattices with beaten egg.

Bake in an oven preheated to 180 degrees until golden brown, about 35 minutes. 

After cooling, Sprinkle it with powdered sugar and then slice it.

A crumbly, fragrant, nutty cake in which the jam and cinnamon complement each other beautifully.