01 - Preheat oven to 350°F. Place unbaked pie crust in a 9-inch pie pan and trim or crimp the edges as desired.
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 - In a mixing bowl, whisk together eggs, granulated sugar, cornstarch, vanilla extract, ground cinnamon, and fine sea salt until smooth.
04 - Gradually pour the warm milk mixture into the egg mixture, whisking constantly to prevent curdling and ensure full incorporation.
05 - Pour custard through a fine-mesh sieve into a clean bowl to achieve a silky texture.
06 - Pour custard filling into the prepared pie shell. Bake 45 to 50 minutes, until the center is set with a slight wobble. Shield crust edges with foil if browning too rapidly.
07 - Remove from oven and cool pie on a wire rack. Custard will finish setting as it cools.
08 - Sprinkle ground cinnamon and optional powdered sugar over the pie before serving. Refrigerate at least 2 hours prior to slicing.