Prepare the base: Mix sugar, cocoa powder, cornstarch, and salt together in a medium saucepan.
Heat the milk: Slowly add the milk to the dry mixture and bring to a boil over medium heat. Boil for 1 minute, stirring constantly.
Temper the eggs: Beat egg yolks in a separate bowl. Add a small amount of the hot milk mixture into the eggs, stirring continuously to prevent cooking.
Combine and thicken: Pour the egg mixture back into the saucepan. Cook for about 3 minutes, stirring constantly, until thickened.
Add flavor and chocolate: Remove from heat and stir in vanilla, chocolate chips, and butter until melted and smooth.
Fill the crust: Pour the chocolate mixture into the pre-baked pie crust while still hot.
Make the meringue: Beat egg whites with a pinch of salt and cream of tartar until stiff peaks form.
Sweeten the meringue: Gradually add sugar and a few drops of vanilla while continuing to beat until glossy.
Top the pie: Spread the meringue over the hot chocolate filling, making sure to seal the edges to the crust.
Bake the pie: Bake at 325°F (163°C) for 20 minutes or until the meringue is golden brown.
Cool and serve: Let the pie cool completely at room temperature to allow it to set before slicing and serving.