In a heavy saucepan, whisk together the sugar, cornstarch, and salt until well mixed.

Slowly whisk in the whole milk, then add the egg yolks until the mixture is smooth.

Place the saucepan over medium heat and stir constantly. Cook until the mixture thickens and begins to bubble, about 8-10 minutes. Remove from heat immediately. Be careful not to overheat, as it can curdle.

Whisk in the butter, vanilla extract, and chopped chocolate until the chocolate and butter melt and the filling is smooth.

Pour the warm chocolate filling into the cooled, pre-baked pie crust. Cover the surface with plastic wrap to prevent a skin from forming. Chill in the refrigerator for at least 2 hours until fully set.

In a clean bowl, beat the heavy whipping cream and powdered sugar until soft peaks form. Spread the whipped cream over the set filling. Garnish with chocolate shavings if desired.