Preheat oven: Preheat oven to 325°F (165°C). Place pie crust into a 9-inch pie pan and crimp edges.
Make the filling: In a saucepan, whisk together sugar, cornstarch, and salt.Gradually whisk in milk and heavy cream until smooth.
Cook until thick: Cook over medium heat, whisking constantly, until mixture thickens (about 6–8 minutes).Remove from heat and stir in butter and vanilla.
Fill the crust: Pour hot filling into the unbaked pie crust.
Add topping: Dot the top with butter pieces. Sprinkle evenly with cinnamon and sugar.
Bake: Bake for 45–50 minutes, until edges are set and center is slightly jiggly.
Cool completely: Let pie cool at room temperature, then refrigerate for at least 4 hours to fully set.