01 - Preheat oven to 350°F. Fit the unbaked pie crust into a 9-inch pie pan and shape the edges as preferred.
02 - Combine whole milk and heavy cream in a medium saucepan. Warm over medium heat until steaming but not boiling, then remove from heat.
03 - Whisk together eggs, granulated sugar, cornstarch, vanilla extract, ground cinnamon, and fine sea salt in a mixing bowl until the mixture is smooth.
04 - Gradually pour the warm milk mixture into the egg mixture while whisking constantly until fully combined.
05 - Pass the custard mixture through a fine-mesh sieve into a clean bowl to remove any lumps.
06 - Pour the strained custard filling into the prepared pie crust.
07 - Bake for 45 to 50 minutes, or until the center is set but slightly wobbly. If the crust darkens too rapidly, loosely cover the edges with aluminum foil.
08 - Transfer the pie to a wire rack and cool completely; the filling will continue to set as it cools.
09 - Sprinkle the top with ground cinnamon and dust with powdered sugar if desired before serving.
10 - Refrigerate for at least 2 hours before slicing for best results.