Preheat oven to 200°C/400°F/Gas Mark 5. Lightly grease a baking tray. On a lightly floured bench, roll out pastry to a rectangle 48 × 24 cm (19 × 9 ½ in). Cut widthways into four equal pieces, 12 × 24 cm (about 5 × 10 in), and brush each piece all over with beaten egg. Place an apple half, cut-side down, at one end of each piece of pastry. Dollop a quarter of the mascarpone on each piece of apple, sit an amaretti biscuit on top, then spinkle with chocolate and half the sugar. Fold pastry over the apple, gently squeezing out any air, and press down well. Cut into a circular shape with a sharp knife. Brush with remaining egg-wash and sprinkle with remaining sugar. Poke a few holes in top with a toothpick and bake for 30 minutes, until puffed and golden.