Combine sugar and cornstarch in a saucepan. Stir in milk until smooth. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir for 2 minutes. Remove from heat; stir in 1 cup into yolks. Return all to pan. Bring to a gentle boil; cook and stir for 2 minutes. Remove from heat; stir in vanilla. Cool slightly. Cover surface of custard with waxed paper; chill. Plac apples in a bowl; sprinkle with lemon juice. Add butter. Combine sugar, cinnamon and cornstarch; mix with apples and set aside. Divide pastry into 8 portions; roll each into a 13 cm square. Spoon filling off-center on each. Brush edges with milk. Fold over to form a triangle; seal. Crimp with tines of fork. Make steam vents in top. Place on greased baking sheets. Chill 15 minutes. Brush with milk. Bake at 200ºC or 35 minutes. Serve warm with custard.