Preheat your oven to 400°F (200°C). Line a 9-inch pie plate with the unbaked crust, pressing it gently into the dish and forming a decorative edge as desired.
Pour the measured water directly into the crust-lined pie plate. Do not stir or mix.
In a separate small bowl, whisk together the flour, sugar, and a pinch of salt until thoroughly combined. Gently distribute this dry blend over the surface of the water in the crust, ensuring even coverage without mixing.
Evenly drizzle the vanilla extract across the top of the filling.
Arrange the butter slices over the filling so they are spaced across the surface. They will melt and spread during baking.
Place the pie carefully in the preheated oven. Bake at 400°F (200°C) for 30 minutes.
Lower the oven temperature to 375°F (190°C) and continue baking for an additional 15 to 20 minutes. The pie is ready when the filling appears set and the edges take on a light golden hue.
Remove from the oven and allow the pie to cool completely at room temperature. The custard-like filling will continue to firm as it cools.
Once cooled and set, slice and serve. Enjoy plain or with a light topping of whipped cream if desired.