1.Make crème pâtissière. Heat milk and vanilla in a small saucepan over medium heat until almost boiling. Meanwhile, whisk yolks, sugar and cornflour in a medium bowl, gradually whisk in hot milk mixture. Return mixture to pan, whisk over low heat 5 minutes or until thickened. Remove from heat, stir in butter. Divide into two small bowls. Stir chocolate into one until smooth, leave remaining bowl plain. Cover surfaces with plastic wrap, refrigerate until firm.
2.Roll out pastry into a 28cm (11¼-inch) square. Cut pastry into three even rectangles, transfer to a baking-paper-lined oven tray. Prick pastry all over with a fork. Freeze 20 minutes.
3.Preheat oven to 220°C (200°C fan forced).
4.Top pastry rectangles with another oven tray (to stop pastry from puffing up). Bake about 15 minutes or until pastry is browned lightly and crisp. Remove top oven tray cool.
5.To make icing, melt butter, whisk butter, sifted icing sugar, glucose and milk in a small bowl (add a little more milk, if needed, to thin to a drizzling consistency). Transfer 1 tablespoon icing to a small bowl, whisk in sifted cocoa and the water. Spoon chocolate icing into zip-top plastic bag. Snip a tiny piece off one corner.
6.Spread plain icing evenly over one pastry rectangle. Pipe chocolate icing crosswise over plain icing, about 1cm (½-inch) apart. Run the tip of wooden skewer down the length of the iced rectangle in alternate directions to create a decorative pattern.
7.Spoon chocolate crème pâtissière into piping bag fitted with a 2cm (¾ inch) plain tube. Place one un-iced pastry rectangle on a serving platter. Pipe chocolate crème pâtissière over the pastry. Top with remaining uniced pastry rectangle, pressing gently to secure. Spoon vanilla crème pâtissière into another piping bag (or clean and dry the bag and tube just used). Pipe crème pâtissière over pastry. Top with iced pastry rectangle, pressing down gently to secure. Refrigerate 1 hour to firm slightly.
8.Cut napoleon into strips using a serrated knife to serve.
You need a piping bag and a 2cm (¾-inch) plain tube; however, you can also spoon the crème pâtissière onto the pastry sheets just give it a quick stir first.Note