For the water pastry, knead all ingredients into a smooth dough, form into a ball and chill in the refrigerator for at least two hours. In the meantime, knead the butter and flour and form into a brick; chill. Wrap the butter pastry in the water pastry and roll it out to about 1 cm (3/8 in.) thick. Fold the dough into thirds (like a business letter), wrap in plastic and chill for an hour. Repeat this entire procedure twice.
Roll the flaky pastry out to about 1.5cm (5/8 in.) and let rest for a few minutes. Cut into squares of 14x14cm (5½x5½ in.). Prick the rest of the dough and cut into smaller squares or circles. Dredge the apple rings in cinnamon sugar and fill the hole of the ring with jam. Brush the edges with egg and bring the corners up to the middle to seal. Brush with egg, top with the pricked dough. Place on a baking tray and brush with egg again.
Baking time: about 45 minutes
Baking temperature: Preheat the conventional oven to 200°C (400°F) and reduce the heat to 180°C (350°F) once the trays are in the oven.
Tip: You can also use purchased puff pastry.