Preheat oven to 200 degrees celsius (fan-forced). Lightly grease 6 holes in a standard muffin tin and set aside.
Place the frozen mixed berries, caster sugar cornflour and vanilla extract into a bowl and mix together until the sugar and cornflour have coated the berries). If using a Thermomix, mix on Reverse, Speed 3, 6 seconds or until combined.
Cut 6 circles from the shortcrust pastry sheets (large enough to cover the base and sides of the muffin holes). Press the circles into the muffin holes.
Fill the muffin holes 3/4 of the way to the top with the berry mixture.
Cut small thin strips of the leftover shortcrust pastry and place over the top of the muffins in a criss-cross pattern. Press the pastry strips into the pastry base.
Brush the pastry with egg and sprinkle over some extra sugar.
Bake for 25 - 30 minutes or until the pastry is golden. Allow to cool slightly before serving with ice-cream or cream.