Preheat oven.to 425°F (220°C).
On a floured surface, roll out pastry dough to a 24x18-inch (60x45-cm) rectangle.
Cut in half lengthwise.
Sprinkle a baking sheet with cold water.
Place rectangles on baking sheet; pierce dough with a fork.
Refrigerate 15 minutes.
Bake 12 to 18 minutes or until puffed and golden.
Cool on a rack.
To make frosting, combine powdered sugar, water and lemon juice in a small bowl.
Spread frosting over one of the cooled pastry pieces.
To make filling, beat egg whites with powdered sugar until stiff and glossy.
Blend cornstarch, egg yolks, granulated sugar, vanilla and a little milk in a medium bowl.
Bring remaining milk to a boil in a medium saucepan; stir hot milk into cornstarch mixture.
Return to saucepan and cook a few minutes, stirring constantly until thickened.
Remove from heat; fold in beaten egg white mixture.
Cool.
Spread cooled filling thickly over piece of pastry without frosting.
Top with frosted pastry.
Refrigerate until set then cut into slices.