Use a blender to process the beans to a smooth paste. Stir the paste, sugar and oil in a saucepan over a medium heat for 10–15 minutes to form a ball. Allow to cool, then roll into 20 small balls.
Make pastry A. Mix the flour, sugar and salt in a bowl. Make a well and add the oil. Gradually add the measured water and knead to a smooth dough. Make into 10 balls and cover with clingfilm.
Make pastry B. Lightly mix the flour and oil in a bowl. Gradually add the measured water and knead until it holds together. Make into 10 balls and cover with clingfilm.
Take a ball of pastry A and flatten it to make a disc. Wrap a ball of pastry B with the disc, then squeeze together. Roll out a rectangle. Take the short edge and roll tightly into a tube. Flatten the pastry lengthways to form a rectangle. Repeat. Roll into a tube and cut in half. Take one half and turn it vertically so it rests on the cut section, then roll it into a round thin sheet. Place a black bean ball in the middle. Press the edges together to seal and place on a lightly oiled baking sheet, sealed side down. Repeat with the remaining pastry and filling until you have 20.
Brush each flaky pearl with egg yolk and bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 25–30 minutes. Serve hot or warm.