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Sieve flour, cornstarch and salt. Combine the egg with a bit of the water and gradually add it to the flour mixture along with the rest of the water. Mix with a whisk or standmixer until you've reached a smooth batter.
The batter should be quite runny. Cover with cling film and set aside for at least half an hour, but preferably overnight.
Stir the batter until it's smooth again and heat a non stick skillet on medium heat. Use a paper kitchen towel to add a bit of oil to the bottom. Divide a thin layer of batter over the bottom, make sure it's not too thick but also not too thin.
If needed you can pour any excess batter back into the bowl, this way you'll end up with an even layer. Cook the wrappers on both sides, theres no need to brown them.
Transfer them to the bottom of a plate or a rack so they can cool off. Repeat for the remaining batter and don't stack them until they've cooled completely.
Now you can fill them with anything you like, such as ham and chicken.