Well ahead of time, prepare two pie dishes with a single crust pastry and pre-bake according to recipe instructions. Allow pie shells to fully cool.
In a medium saucepan, bring the water to a boil.
Meanwhile, combine the eggs, sugar, flour, salt, and vinegar in a mixing bowl and use a whisk to combine.
Pour the filling mixture into the boiling water and continue to boil, whisking constantly, until mixture thickens. (6-12 minutes, depending on amount of water.)
When mixture thickly coats the back of a spoon, remove the pan from heat.
Add in the cinnamon and maple extract and stir to combine.
Pour mixture through a sieve into each pie crust to prevent any curdled egg from ending up in the finished pie.
Refrigerate for 4+ hours (preferably overnight) before serving.