1.Preheat oven to 200ºC. Grease three 12-hole (1-tablespoon/20ml) mini muffin pans.
2.Stir 300g frozen mixed berries and 1/4 cup caster (superfine) sugar in small saucepan over low heat until sugar dissolves, bring to the boil.
3.Blend 2 teaspoons cornflour (cornstarch) with 1 tablespoon water, stir into berry mixture. Stir over heat until mixture boils and thickens. Cool.
4.Cut 36 x 6cm rounds and 36 x 4cm rounds from shortcrust pastry, press large pastry rounds into pan holes. Spoon berry mixture into pastry cases, top with small pastry rounds. Press edges firmly to seal. Brush tops with 1 lightly beaten egg white, sprinkle with 1 tablespoon caster (superfine) sugar. Make small cut in top of each pie. Bake pies about 20 minutes. Stand in pan 10 minutes before turning, top-side up, onto wire rack.