Whisk the flour, baking powder, salt, and powdered sugar in a bowl. Rub in the cold butter with your fingertips for 2 to 3 minutes, until the mix looks like coarse crumbs with a few pea-size bits. Add the dough yolk and sour cream, then mix with a fork and finish by hand for about 1 minute, just until the dough holds together. It will be soft. Shape it into a disk, wrap it, and chill for 30 minutes.

While the dough chills, stir the almonds, sugar, cardamom, and water in a small bowl until the filling clumps when you squeeze it. Divide it into 18 portions of about 15 g (1 level tablespoon each) and roll each into a ball.

Heat the oven to 180 C / 350 F and line two 33 x 46 cm / 13 x 18 inch sheet pans with parchment. Roll half the dough on a lightly floured surface to 4 mm thick, keeping the other half in the fridge. Cut 9 cm / 3.5-inch rounds with the cutter. Re-roll the scraps once.

Set a filling ball in the center of each round and fold the dough over into a half-moon. Press the edge shut with your thumb, pushing out trapped air. Pinch a pattern along the curved edge with clean tweezers or press fork tines along the seam. Place the pastries 2.5 cm / 1 inch apart on the pans, about 9 per pan.

Whisk the glaze yolk with the milk and brush a thin layer over each pastry. Bake both pans for 18 to 20 minutes, swapping their positions at the 10-minute mark, until the bottoms and edges are light golden brown.

Cool the shekerbura on the pans for 10 minutes, then move them to a wire rack. Serve warm or at room temperature.