Roll your sheets of pastry very thin. You are looking for 2 squares/rectangles that are approximately 30cm x 30cm each. Prick them well all over and then place on baking sheets (roll on baking parchment and then slide the whole lot onto the baking sheet for ease of use) and bake at 210C for 10 - 12 minutes (or until golden brown and cooked through). Leave the pastry sheets to cool while you make the custard.
Put 2 Cups of milk into a pan with the butter and heat to melt.
Mix the cornflour, icing sugar and eggs to a smooth paste with the remaining 1/2 cup of milk.
Remove the milk and butter from the heat and whisk in the egg mixture then return to the heat.
Cook over medium heat until the mixture thickens and then for a further 5 minutes, stirring constantly (a wooden spoon works well here). Remove from the heat and stir in the vanilla. Leave the custard until cool.
Place one sheet of pastry on a piece of baking parchment on a tray or board and cover with all the cool custard. Place the other pastry sheet on top and press down slightly until both sheets have adhered to the pastry.
Sift the cocoa and icing sugar together, then stir in the milk to make a smooth, spreadable icing.
Ice the whole lot with Chocolate icing and then place in the fridge for a couple of hours so that the whole thing is a little easier to manage. Cut into squares with a serrated knife and serve. Store any remaining squares in the fridge in an airtight container (don't worry, they won't last long!).