1. In a large bowl, combine the kefir, sugar, and vegetable oil. Mix well until the sugar dissolves. 2. Combine the sifted flour with the baking powder and gradually add to the wet mixture. Knead into a soft, smooth dough that doesn't stick to your hands. 3. Roll out the dough to about 1 cm thickness and cut out cookie shapes using a glass or cookie cutters. 4. Place the cookies on a baking sheet lined with parchment paper. Brush the tops with egg yolk and sprinkle lightly with sugar. 5. Bake in a preheated oven at 180°C for about 15-20 minutes until beautifully golden brown.