Heat the oven to 375°F. Fit the pastry crust into a 9-inch pie plate, crimp the edge, and prick the bottom with a fork. Line the crust with parchment, add pie weights or dried beans, and bake for about 15 minutes. Remove the weights and parchment, then bake for another 5–10 minutes until lightly golden. Cool completely.

Whisk the sugar, cornstarch, and salt together in a saucepan. Whisk in the whole milk and egg yolks until the mixture is smooth and evenly blended.

Cook the custard over medium heat, whisking constantly, until it thickens and begins to bubble. Once bubbling, continue cooking for about 1 minute so the cornstarch fully activates.

Remove the saucepan from the heat and whisk in the butter and vanilla extract. For the smoothest filling, pass the warm pastry cream through a fine-mesh sieve into a clean bowl.

Slice the bananas just before assembling the pie. Arrange about half of the banana slices over the cooled crust, spread the warm or slightly cooled pastry cream over them, and add the remaining banana slices on top or reserve them for garnish.

Press plastic wrap directly against the surface of the custard and refrigerate for at least 4 hours, preferably overnight. Just before serving, whip the cold heavy cream, powdered sugar, and vanilla to medium-stiff peaks, then spread or pipe it over the chilled pie.