In a large mixing bowl, combine the all-purpose flour and salt (and sugar if using). Add in the cold unsalted butter cubes and mix until it resembles coarse crumbs. Gradually add ice water, one tablespoon at a time, mixing until the dough holds together without being sticky.

Shape the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

Preheat your oven to 375°F (190°C). Roll out your dough on a lightly floured surface until it's about 1/8 inch thick. Transfer it to a pie dish and trim any excess edges.

Blind bake the crust by placing parchment paper over it and filling it with pie weights or dried beans. Bake in preheated oven for about 15 minutes until lightly golden.

In another bowl, whisk together three large eggs and granulated sugar until smooth. Then gently stir in heavy cream, pure vanilla extract, ground cinnamon, and salt until well combined.

Remove your crust from the oven and carefully pour in the filling mixture. Return it to the oven and bake for approximately 35 minutes or until set but still slightly jiggly in the center.

Let your custard pie cool at room temperature before refrigerating for at least two hours (or overnight if you can wait!). Serve chilled topped with whipped cream or an extra sprinkle of cinnamon.