Preheat the oven to 200°C. Line a large baking tray with baking paper and set aside

In a medium saucepan, heat the butter over a medium heat. Once the butter has melted, add the apple, sultanas, and lemon zest. Stir gently for 5 minutes or until the apples have softened. Set aside

Lay out the pastry and cut each square into 4. Place 1 tbsp of the apple into the middle of each square, and sprinkle with 1 tsp of caster sugar. Brush the edges lightly with water. Fold one corner over to the opposite corner, and press the edges together. Transfer onto the prepared tray, and make a small piercing in each turnover to allow steam to escape

Mix the egg yolk and milk together, and brush over the pastry using a pastry brush. Sprinkle over the remaining sugar and place into the oven to bake for 15-20 minutes or until puffed, golden and crisp. Remove from oven and allow to stand for 5-10 minutes before serving