Preheat your oven to 350°F (175°C).

In a mixing bowl, combine graham cracker crumbs, melted butter, and sugar until well-blended. Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish. Bake the crust for about 10 minutes, then remove and allow to cool.

In a double boiler or microwave-safe bowl, whisk together sugar, cocoa powder, cornstarch, and salt. Gradually add the milk while whisking continuously to prevent lumps.

Cook the mixture over medium heat or in the microwave until it thickens and bubbles, about 10-15 minutes. Stir frequently to ensure even cooking.

In a separate bowl, lightly beat the egg yolks. Gradually whisk in a small amount of the hot chocolate mixture to temper the yolks. Then, combine the tempered yolks back into the saucepan with the rest of the chocolate mixture.

Cook for an additional 2-3 minutes, stirring constantly. Remove from the heat and stir in chopped chocolate, butter, and vanilla extract until smooth. Pour the chocolate filling into the cooled crust.

In a clean mixing bowl, beat the egg whites and cream of tartar until soft peaks form. Gradually add the sugar and continue to beat until stiff peaks form. Mix in the vanilla extract.

Spread the meringue over the chocolate filling, sealing it to the edges of the crust to prevent shrinking. Use a spatula to create peaks and swirls on the meringue.

Bake the pie in the preheated oven for 10-15 minutes or until the meringue is golden brown. Keep an eye on it to prevent burning.

Once done, remove from the oven and cool. Refrigerate for at least 2 hours before serving to set the filling.