In a medium saucepan, whisk together 1 cup (200 grams) sugar, flour, and salt. Slowly whisk in milk. Cook over medium heat, whisking constantly, until mixture is bubbly and thickened. Cook 2 minutes more.
Place egg yolks in a medium bowl. Slowly drizzle 1⁄2 cup hot milk mixture into egg yolks, whisking constantly. Whisk in remaining hot milk mixture. Return mixture to saucepan, and cook, stirring constantly, until mixture thickens, 5 to 10 minutes. Remove from heat; add chocolate, butter, and 1 1⁄2 teaspoons (6 grams) vanilla, stirring until smooth. Pour filling into prepared Chocolate Crust. Cover with a piece of plastic wrap, pressing wrap directly onto surface of filling to prevent a skin from forming. Refrigerate overnight, or freeze for at least 3 hours.
In the top of a double boiler, whisk together egg whites, cream of tartar, remaining 1 cup (200 grams) sugar, and remaining 1 teaspoon (4 grams) vanilla. Cook over simmering water, stirring constantly with a spatula, until sugar is dissolved, about 6 minutes. Transfer mixture to the bowl of a stand mixer fitted with the whisk attachment; beat at high speed until medium- stiff peaks form, about 5 minutes. Fold in cocoa. Place large dollops of meringue on top of pie. Spread meringue just barely to edges of crust to prevent weeping. Using a kitchen torch, carefully brown meringue, or bake in a 500°F (260°C) oven until topping is browned, about 5 minutes. Serve immediately.
Recipe by Bake from Scratch at https://staging.bakefromscratch.com/triple-chocolate-pie/