Heat the oven to 200C/fan 180C/gas 6. Unroll the puff pastry sheet and use a knife to lightly score a border 4cm from the edge all the way around. Use a fork to prick the inside all over, then slide the whole sheet onto a baking-paper-lined tray. Brush with beaten egg and lightly dust with icing sugar. Bake in the oven for 15-20 minutes or until golden and crisp, and the sides have risen. Remove from the oven and, once cooled slightly, gently push down the middle if it has risen. Cool completely.