To make filling, strain cherries over a bowl and reserve 2 tablespoons of the cherry liquid. Put sugar and lemon juice in a saucepan over a medium heat and stir until sugar begins to dissolve.
Add cherries to pan and stir. Bring to a simmer. Combine cornflour and reserved cherry liquid until smooth. Stir into cherry mixture until it boils and thickens. Set aside to cool.
To make sweet shortcrust pastry, put butter, flour and icing sugar mixture in the bowl of a food processor. Process until mixture resembles breadcrumbs. Add yolks and water, then process until mixture begins to form a ball. Turn out onto a lightly floured surface. Knead until smooth. Cut dough in half, enclose pieces in plastic wrap and refrigerate for 15 minutes.
Preheat oven to 180°C. Line 2 oven trays with baking paper. On a floured surface, roll out 1 piece of pastry until 3mm thick. Using a 9.5cm pastry cutter, cut out as many circles as you can. Gather pastry scraps, without excess flour, knead into a ball and roll out again. Cut out more circles. Repeat with remaining pastry.
Brush edges of 3 pastry circles with beaten egg. Put 1 heaped tablespoon of cherry mixture in centre of each. Top with pastry circles. Press edges to seal, making sure filling doesn’t ooze out. Repeat with remaining pastry circles and cherry mixture.
Put pies on prepared trays. Cut a small cross in the top of each and brush with remaining egg. Sprinkle with extra sugar. Bake for 20-25 minutes or until golden. Serve.