Preheat your oven to 350°F (175°C).

In a mixing bowl, combine graham cracker crumbs, melted butter, and sugar for the crust. Mix well until the crumbs are evenly coated.

Press the mixture into a 9-inch pie dish to form an even layer along the bottom and up the sides. Bake in the preheated oven for 10 minutes. Remove and let cool.

In a saucepan, whisk together the sugar, cocoa powder, cornstarch, and salt for the filling. Gradually add the milk, stirring constantly until the mixture is smooth.

Place the saucepan over medium heat and cook, stirring frequently, until it thickens and comes to a gentle boil, about 10-15 minutes.

Once thickened, remove from heat. Stir a small amount of the hot filling into the egg yolks to temper them, then gradually add the yolk mixture back into the saucepan. Cook for an additional 2 minutes while stirring, then remove from heat.

Stir in the butter and vanilla extract until fully incorporated.

Pour the chocolate filling into the cooled crust and spread it evenly.

For the meringue, beat the egg whites with cream of tartar in a clean bowl until soft peaks form. Gradually add sugar, continuing to beat until stiff peaks form. Mix in the vanilla extract.

Spread the meringue over the chocolate filling, making sure to seal the edges well to prevent shrinking.

Bake in the oven for about 10-15 minutes or until the meringue is golden brown.

Allow the pie to cool at room temperature for at least 2 hours before slicing and serving.