Preheat the oven to 200°C/400°F/Gas mark 6. Lightly grease a non-stick baking tray.
Roll out the puff pastry to a thickness of 1cm/1⁄2in, then trim and cut into 2 equal rectangles. Put one on the baking tray.
Peel, core and dice the apple. Fry in the butter. When golden and starting to soften, add the muscovado sugar and toss to coat. Remove from the heat and stir in the sultanas, mixed peel and allspice.
Spread the mixture onto the pastry rectangle on the baking tray. Put the other rectangle on top, and press down lightly. Divide into 16 squares with a sharp knife, pressing nearly all the way through, and brush the tops with egg yolk.
Bake in the oven for 15–20 minutes. Transfer to a wire rack to cool. Dust with icing sugar.