Roll out the pie crust and fit it into a pie plate. Crimp the edges. Blind bake at 375°F (190°C) for 15-20 minutes with pie weights, then 5-10 minutes without weights until golden brown. Let cool.

In a saucepan, whisk together sugar, cocoa powder, cornstarch, and salt.

Gradually whisk in milk until smooth.

Cook over medium heat, stirring constantly, until the mixture thickens and bubbles.

Reduce heat to low and cook for 1-2 minutes, stirring constantly, until thick like pudding.

Remove from heat and stir in butter, vanilla extract, and chocolate chips (if using) until melted and smooth.

Pour the hot filling into the prepared pie crust.

Press plastic wrap onto the surface of the filling and refrigerate for at least 3 hours, or overnight, to set.