Combine sugar and corn starch 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.
Place apples in a bowl; sprinkle with lemon juice. Add butter.
Combine sugar, cinnamon, and corn starch; mix with apples and set
aside. Divide pastry into 8 portions; roll each into a 5" 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 400 F for 35 minutes. Serve warm with custard.
