Let store-bought crust come to room temperature or roll out homemade crust with flour. Fit into 9-inch pie pan and trim edges. Crimp decoratively. Prick bottom with fork several times. Refrigerate while making filling. Cold crust helps prevent soggy bottoms.

Whisk eggs and sugar together in large bowl until smooth. Don't beat too hard or you get bubbles. Heat milk and heavy cream in saucepan until steaming but not boiling. Remove from heat and let cool for 2 minutes. This next part is crucial.

Here's where most people wreck their cinnamon vanilla custard pie. Slowly drizzle ½ cup hot milk mixture into eggs while whisking constantly. Keep whisking and add another ½ cup slowly. This gradually raises egg temperature without scrambling them. Once eggs are warmed, pour them into remaining milk mixture. Whisk in vanilla extract, ground cinnamon, nutmeg, salt, and melted butter. Strain through fine-mesh sieve to remove any egg bits.

Preheat oven to 325°F - low and slow is the secret. Place chilled pie crust on baking sheet for easier handling. Pour custard filling into unbaked crust. Sprinkle extra cinnamon on top. Carefully transfer to oven without sloshing. Bake for 45-55 minutes until edges are set but center still jiggles slightly. The center should wobble like jello, not be completely firm.

Remove from oven when center still has that slight jiggle. Place on wire rack and let cool at room temperature for 2 hours. The custard will continue setting as it cools. Don't rush this step or you'll have soup in a pie shell. Once cooled, refrigerate for at least 2 hours before serving. Custard pies are best served chilled.