Make the pastries

• Rub the 100 g dough butter into the 260 g flour, baking powder and salt with your fingertips until the mixture looks like fine crumbs.
• Mix in the kefir and whole egg just until the mixture comes together as a soft dough.
• Wrap the dough in plastic wrap and refrigerate for 30 minutes.
• While the dough chills, melt the 50 g filling butter in a heatproof bowl over a saucepan of hot water. Let it cool, then stir in the sugar and 60 g flour to make an evenly mixed, spreadable paste.
• Preheat the oven to 180°C (350°F).
• Divide the chilled dough in half and roll each half on a lightly floured worktop into a rectangle about 20 × 25 cm and 0.5 cm thick, keeping the thickness even.
• Spread half the filling in a thin, even layer over each sheet, leaving a narrow strip of bare dough along one long edge.
• Roll each sheet snugly toward the bare edge to form a log, then pinch the seam closed and place it seam-side down.
• Brush the tops of both logs evenly with the beaten egg yolk.
• Gently press shallow lines along the tops of the logs with the back of a knife.
• Cut each log into 7–8 pieces about 3 cm wide, about 15 pastries in all, and place them on the lined tray with gaps between them.
• Bake for about 20 minutes, until golden on top and a wooden skewer inserted into the pastry comes out without raw dough.
• Let the pastries cool slightly on the tray before serving.