Toast the walnuts in a dry skillet over medium heat for 3-4 minutes until fragrant, then let them cool completely. Chop the toasted walnuts into small pieces.

In a mixing bowl, combine the chopped walnuts with sugar, egg whites, and apricot jam. Mix thoroughly until all ingredients are well incorporated. Set aside.

In a large mixing bowl, combine the melted butter with powdered sugar and stir until smooth. Add the egg yolk, vanilla sugar, and room temperature sour cream or yogurt. Mix well until the mixture is creamy and uniform.

In a separate bowl, whisk together the flour and baking powder. Gradually add the flour mixture to the wet ingredients, mixing continuously until you achieve a soft, pliable dough.

Shape the dough into a ball, wrap it in plastic wrap, and refrigerate for 30 minutes.

Remove the dough from the refrigerator and divide it into 3 portions, with one portion slightly larger than the others. Roll out the largest portion on a lightly floured surface to fit a 22 cm (8.5 inch) round cake pan.

Place the first rolled dough portion into the prepared pan, pressing it gently against the bottom and sides. Spread half of the walnut filling evenly over this base layer.

Roll out the second portion of dough to the same size and place it over the walnut filling. Press the edges together to seal.

Spread the remaining walnut filling over this layer and roll out the third portion of dough to cover it. Seal all edges firmly.

Beat the remaining egg yolk with a teaspoon of water. Brush this egg wash over the top of the cake for a golden finish.

Preheat your oven to 180°C (350°F). Bake the cake for 40-45 minutes until deeply golden brown. Allow to cool completely before unmolding and slicing.