Preheat the oven to 375°F (190°C). Fit the unbaked pie crust into a 9-inch pie dish, crimp the edges, and prick the bottom lightly with a fork. Line with parchment paper and fill with pie weights. Bake for 15 minutes, then remove the parchment and weights and bake for another 8 to 12 minutes, until the crust is golden brown. Cool completely.

In a medium saucepan, whisk together the granulated sugar, cornstarch, 1 1/2 teaspoons cinnamon, and salt. Gradually whisk in 1/2 cup (120 mL) of the milk until smooth with no visible cornstarch lumps.

Whisk the egg yolks into the saucepan mixture: Gradually whisk in the remaining 2 cups (480 mL) milk until completely combined.

Place the saucepan over medium heat and cook, whisking constantly, for about 8 to 10 minutes. The mixture will gradually thicken. Once it reaches a full, gentle boil, continue cooking for about 1 minute, whisking continuously. The custard should be thick enough to coat a spoon and hold a clear line when you run a finger through it.

Remove the saucepan from the heat: Stir in the butter and vanilla until completely melted and smooth. If the custard has any small lumps, pour it through a fine-mesh strainer into a heatproof bowl.

Pour the warm custard into the completely cooled pie crust. Smooth the surface with a rubber spatula. Immediately press plastic wrap directly against the surface of the filling to prevent a skin from forming.

Just before serving, beat the cold heavy cream, powdered sugar, and vanilla with an electric mixer on medium-high speed until medium peaks form. The cream should hold its shape but still look smooth rather than grainy.

Spread or pipe the whipped cream over the chilled pie. Sprinkle the remaining 1/2 teaspoon cinnamon over the whipped cream. Slice with a sharp knife and serve cold.