Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the crushed chocolate cookies and melted butter for the crust. Press the mixture evenly into the bottom and sides of the pie dish to form the crust.
In a saucepan, whisk together the granulated sugar, flour, cocoa powder, and salt for the chocolate filling. Gradually stir in the milk until the mixture is smooth.
Place the saucepan over medium heat and cook the mixture, stirring constantly, until it thickens and comes to a boil. Let it boil for one minute, then remove it from the heat.
In a separate bowl, lightly beat the egg yolks. Slowly pour about 1 cup of the hot chocolate mixture into the beaten yolks, whisking constantly to prevent curdling. Pour the egg mixture back into the saucepan and return it to medium heat.
Cook the mixture, stirring constantly, for an additional 2 minutes. Take the mixture off the heat and stir in the butter and vanilla extract, ensuring that the butter is completely melted.
Pour the chocolate filling into the prepared crust and set aside.
In a clean mixing bowl, beat the egg whites and cream of tartar using an electric mixer until soft peaks form. Gradually add the granulated sugar while continuing to beat until stiff, glossy peaks form.
Spread the meringue over the chocolate filling, ensuring that it touches the edges of the crust to seal it completely.
Bake the pie in the preheated oven for about 12-15 minutes, or until the meringue is golden brown.
Remove the pie from the oven and let it cool completely on a wire rack before serving.