Crust
Streusel
Filling
Crust
Mix the flour with sugar and baking powder, then add the cold butter. Next, add the egg yolk and sour cream, and knead into a dough. Place the dough in a plastic bag and refrigerate for one hour.
Streusel
In a bowl, combine the flour, walnuts, and sugar, then add the butter and rub together to make a streusel. Refrigerate for one hour.
Preheat the oven to 180°C. Line a round baking tray with parchment paper. Press the dough evenly into the bottom of the pan and prick it with a fork. Bake for 15 minutes.
Remove from the oven, spread the Lauretta filling on top, and sprinkle with the streusel, pressing it down lightly.
Bake for 35 minutes.