DO CRUST. In a bowl, combine crushed Grahams and butter. Press onto the bottom of an 8x8 pan. Chill.
DO CUSTARD. Mix sugar and cornstarch in a saucepan. Gradually stir in milk. Cook over medium heat, stirring constantly, until thick. Gradually stir in half of the warm milk mixture into beaten egg yolks, then stir back into saucepan with remaining milk mixture. Continue to cook until thick. Remove from heat. Stir in butter and vanilla.
LAYER. Pour custard into prepared piecrust and top with mango slices or balls. Heat mango jam until melted and brush over mangoes.