Prepare the crust. Stir the graham cracker crumbs, melted butter, and sugar together until evenly moistened. Press firmly into the bottom and sides of a 9-inch pie plate, then refrigerate while you prepare the filling.

Mix the custard base. Whisk the sugar, cornstarch, and salt together in a saucepan. Slowly pour in the cold milk, whisking continuously until smooth.

Thicken the filling. Add the egg yolks and place the saucepan over medium heat. Cook for about 6 to 8 minutes, stirring constantly, until the custard thickens and begins to bubble gently.

Finish the walnut custard. Remove the pan from the heat and immediately stir in the butter, vanilla, and chopped walnuts. Continue stirring until the butter has completely melted and the filling is smooth.

Fill and chill. Spoon the warm custard into the chilled graham cracker crust and smooth the surface. Cool for about 20 minutes at room temperature, then refrigerate for at least 2 hours or until fully set.

Prepare the whipped topping. Beat the heavy cream, powdered sugar, and vanilla until soft peaks form.

Decorate the pie. Spread or pipe the whipped cream over the cold pie and finish with the remaining chopped walnuts.

Serve chilled. Slice with a clean knife and serve straight from the refrigerator for the smoothest, creamiest texture.