Preheat the oven to 350°F (175°C).

Combine the graham cracker crumbs, sugar, and melted butter in a mixing bowl. Mix until fully combined. Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish. Bake for 8-10 minutes, then remove from the oven and let it cool.

In a medium saucepan, whisk together the sugar, cocoa powder, cornstarch, and salt. Gradually add the milk while whisking until smooth. Cook over medium heat, stirring constantly, until the mixture begins to thicken and bubble, about 5-7 minutes.

In a separate bowl, lightly beat the egg yolks. Slowly whisk about 1 cup of the hot chocolate mixture into the yolks to temper them, then add the egg yolk mixture back into the saucepan. Cook for another 2-3 minutes until thickened.

Remove the saucepan from heat and stir in the butter and vanilla extract until melted and smooth. Pour the chocolate filling into the cooled pie crust.

In a mixing bowl, beat the egg whites and cream of tartar until soft peaks form. Gradually add the sugar while beating until stiff peaks form and the mixture is glossy.

Spread the meringue over the chocolate filling, making sure to seal the edges to prevent shrinking. Create small peaks with a spatula for a decorative touch.

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

Remove the pie from the oven and let it cool at room temperature for at least 1 hour before slicing.

For best results, let the pie cool completely in the refrigerator for a few hours before serving.