01 - Preheat oven to 350°F. Fit the unbaked pie crust into a 9-inch pie pan, crimping or trimming edges for an even finish.
02 - Combine whole milk and heavy cream in a medium saucepan. Warm gently over medium heat until steaming, without bringing to a boil. Remove from heat.
03 - In a mixing bowl, briskly whisk eggs, granulated sugar, cornstarch, vanilla extract, ground cinnamon, and sea salt until the mixture is fully blended and smooth.
04 - Gradually pour the warmed dairy into the egg mixture, whisking continuously to temper the eggs and integrate ingredients thoroughly.
05 - Pour the custard mixture through a fine-mesh sieve into a clean bowl to remove any curdled particles, ensuring a silky texture.
06 - Transfer the smooth custard filling into the prepared pie crust. Bake for 45 to 50 minutes, or until the center is set but still slightly wobbles. Shield crust edges with foil if overbrowning.
07 - Cool the pie on a wire rack. The custard will continue to firm as it cools. Refrigerate for at least 2 hours before slicing.
08 - Before serving, sprinkle the surface with ground cinnamon and dust lightly with powdered sugar if desired.