Preheat your oven to 375°F (190°C). Roll out the pie crust into a 9-inch pie dish; line with parchment paper and fill with weights. Bake for 15 minutes, remove weights, and bake for another 5 minutes until golden. Allow to cool slightly.

In a medium saucepan, combine whole milk, heavy cream, ground cinnamon, and vanilla extract over medium heat. Warm until it steams but do not boil. Remove from heat and steep for 10 minutes if using a cinnamon stick or vanilla bean.

In a mixing bowl, whisk together 4 eggs and ¾ cup of sugar until pale and smooth, about 2-3 minutes.

Slowly pour the warm milk mixture into the egg-sugar mixture while whisking to prevent eggs from scrambling. Strain the mixture through a fine-mesh sieve into a large bowl.

Lower oven temperature to 325°F (160°C) and pour custard into the pie crust. Bake for 35-40 minutes until edges set but center jiggles slightly.

Remove from oven and let cool at room temperature for 1 hour. Cover and chill in the refrigerator for at least 2 hours before serving to deepen flavors.