1.Process flour, icing sugar and butter until mixture forms coarse crumbs. Add egg yolk and enough of the chilled water to make ingredients just come together. Gently knead dough on a floured surface until smooth. Press into a flat square. Wrap in plastic wrap; refrigerate for 30 minutes.
2.Meanwhile, make strawberry filling. Cook strawberries, jam and juice in a small saucepan over medium-low heat, stirring occasionally, for 4 minutes or until strawberries soften. Add cornflour; cook, stirring, for 1 minute or until thick. Transfer to a heatproof bowl; cool 10 minutes. Cover; refrigerate until needed.
3.Preheat oven to 210°C. Line two large oven trays with baking paper. Cut pastry in half. Roll one half between sheets of baking paper into a 20cm x 24cm rectangle. Cut in half lengthways, then cut each half into four 6cm x 10cm rectangles. Repeat with remaining pastry.
4.Spread 3 teaspoons strawberry filling onto each of eight pastry rectangles, leaving a 1cm border. Brush edges lightly with egg. Top with remaining pastry rectangles; press together with a fork to seal. Place tarts on tray. Prick with a fork to allow steam to escape. Brush with a little egg; sprinkle with demerara sugar. Refrigerate for 30 minutes.
5.Bake tarts for 15 minutes or until golden. Cool on trays for 10 minutes before transferring to a wire rack to cool completely.
6.Sift extra icing sugar into a medium bowl; stir in enough of the extra water to make icing a spreadable consistency. Tint pale pink with food colouring. Spread icing over tarts; decorate with sprinkles. Stand until set.
You could use blueberries or raspberries in the filling instead of strawberries, if you like.
Tarts can be made a day ahead; store in an airtight container.