Preheat the oven to 400F (205C).

In a saucepan, melt the butter, add the flour and salt, and cook for 1 minute.
Remove from heat, continue stirring, add the water and then the sugar.
Continue cooking over medium heat, stirring until a smooth and thick cream is obtained.
Add the apples, cover, and cook over low heat for 15 minutes, stirring often during cooking. Remove from heat, add the vanilla extract, and let cool.
Line a 9-inch (23 cm) tart mold with a crust, pour in the filling.
Cover with the second crust and seal the edges well.
Make incisions on top so that the steam can escape.
Bake in the oven for about 30 to 35 minutes.
Let cool before serving.