Add the sugar, cornstarch, cocoa and salt to a medium-sized saucepan (off the heat). Whisk until combined and no lumps of cornstarch remain. Add the egg yolks and whisk until it forms a thick paste.

Add milk and whisk until everything is completely dissolved and smooth. Move the saucepan to the stove and cook over medium heat, stirring constantly with a whisk until liquid thickens and comes to a bubble. Stir continually or it will scorch.

Once the mixture boils and the first bubble rises to the surface, reduce heat to low and continue to cook, stirring constantly, for 1 minute exactly.

Remove from heat and whisk in chopped chocolate. Stir until completely melted and smooth then add vanilla and butter and stir until all butter is melted and well-incorporated.

Pour filling into a bowl and cover with plastic wrap, making sure to place the plastic wrap directly in contact with the filling to prevent a skin from forming on the surface. Refrigerate at least 3 hours or until cool to the touch.

Remove cooled filling from refrigerator and whisk until smooth then spoon into prepared pie crust. Refrigerate until set and top with whipped cream and chocolate sprinkles if desired.