Make the DoughIn a large mixing bowl, combine the all-purpose flour and a pinch of salt. Add the softened butter and work it into the flour using your fingertips until the mixture resembles coarse crumbs. Add the egg yolks and sour cream, mixing until a smooth, soft dough forms.

Divide and ChillShape the dough into a log and divide it into 30 equal portions. Roll each into a ball, then flatten slightly into a disc. Place the discs on a tray, cover, and refrigerate for 30 minutes to firm up.

Prepare the MeringueIn a clean mixing bowl, whisk the egg whites until foamy. Add the vanilla sugar, granulated sugar, and lemon juice. Continue whisking for 8–10 minutes until the meringue forms stiff, glossy peaks. Sprinkle in the cornstarch and fold gently until incorporated.

Shape the CookiesPreheat the oven to 200°C (390°F). Roll out each dough disc into a thin round, about 6 cm in diameter. Spoon a dollop of meringue into the center, then gently fold over another disc to create a sandwich, sealing the edges lightly.

BakeArrange the cookies on a parchment-lined baking sheet and bake for 10–15 minutes, or until lightly golden around the edges.

Cool and DustAllow the cookies to cool slightly on the tray before transferring to a wire rack. Once cooled, dust generously with powdered sugar.