Make Pastry: Sift flours, custard powder and sugar into a bowl. Whisk to combine, add butter then transfer ingredients to the bowl of a food processor. Process until mixture resembles fine breadcrumbs. Add egg and process again. Check consistency of pastry dough. If very wet add just 1 teaspoon of iced water. If dry, add more water until the dough is only slightly crumbly. Process until dough just comes together. Turn pastry out onto a large piece of baking paper, and bring it together gently – too much handling will result in a tough, chewy pastry. Shape into a disc and cover in plastic wrap. Refrigerate for 30 minutes.
Meanwhile, combine cherries, lemon juice, cinnamon and sugar in a medium saucepan over medium heat. Cook, stirring, for 5 minutes or until cherries have thawed and sugar is dissolved. Blend cornflour with 1 tablespoon cold water in a small bowl. Stir into cherry mixture. Cook, stirring, for 2 to 3 minutes or until mixture boils and thickens. If using unfrozen cherries, add 1-2 more tablespoons of water. Cool completely.
Preheat oven to 180C (350F) fan-forced. Place an oven tray in oven. Grease a 4cm-deep, 20cm (base) round pie dish (or use a tin foil pie dish). Roll out two-thirds of the pastry between 2 sheets of baking paper until large enough to line base and side of prepared dish (about 30cm). Line dish with pastry. Trim edge. Refrigerate for 15 minutes.
Spoon cooled cherry mixture into pastry base. Roll out remaining pastry between 2 sheets baking paper, to form a 25cm-round. Peel off one layer of baking paper and use the other one to help guide the pastry over the top of the pie. Once in place, peel the remaining paper off and crimp the pasty edges with your fore finger and thumb press to seal. Poke pastry with a fork several times and brush pastry top with a little egg and milk mixture. Sprinkle with extra sugar. Place pie dish on heated tray in oven. Bake for 40 to 45 minutes or until pastry is golden and crisp (cover pastry edge with foil if over-browning). Stand for at least 20 minutes before cutting.