Start by preheating the oven to 350 degrees F (175 degrees C). Grease a baking tray using butter or oil, ensuring complete coverage, and set aside.
Take the puff pastry sheets and carefully cut them into small rectangles of equal size. Place these rectangular pieces on a greased baking tray.
In a medium saucepan, combine the butter, cornflour and sugar. Stir the ingredients together well. Gradually add the milk to the mixture and whisk constantly over medium heat until a thick and creamy consistency is achieved.
In a separate bowl, beat two eggs and add to the saucepan mixture. Keep stirring until the mixture is smooth and well blended.
Pour this creamy mixture evenly over the puff pastry rectangles, let it cool.
In a separate bowl, combine the chocolate chips and beaten egg to make a Cheshire cat tail batter. Spread this mixture evenly on each puff pastry rectangle and shape it as desired.
Place the baking tray with the prepared rectangles in the preheated oven and bake for about 20 minutes or until they are golden brown.
While the rectangles are baking, prepare the icing for the tails. In a small bowl, combine 4 tablespoons milk with ¼ cup powdered sugar. Divide the mixture into two portions and add food colouring to each portion, making different coloured icings.
Once the rectangles have finished baking, drizzle one half with white icing and the other half with coloured icing.
Transfer the finished pastries to a plate and serve immediately to enjoy their delicious flavours.