1 9-inch pre-baked pie shell
10 tablespoons plus 1/2 cup granulated sugar, divided
1/4 cup, plus 1 tablespoon cornstarch, divided
2 tablespoons unsweetened cocoa powder
5 large eggs, separated
1 can (12 ounces) evaporated milk
1 cup whole or 2% milk
4 ounces bitter- or semi-sweet chocolate, coarsely chopped or broken
2 tablespoons butter
1 1/2 teaspoons vanilla extract
1/4 teaspoon cream of tartar
Prepare and bake pie shell; set aside.
Meanwhile, adjust oven rack to lower-middle position and heat oven to 325 degrees. Whisk 10 tablespoons of the sugar, 1/4 cup of the cornstarch, cocoa powder, and salt in a medium saucepan. Whisk in yolks, and then immediately, but gradually whisk in milks. Cook over medium heat, stirring frequently at first, and then constantly as mixture starts to thicken and begins to sputter, 8 to 10 minutes. Whisk chocolate and butter into thickened pudding until melted, about 1 minute longer; off heat stir in 1 teaspoon of the vanilla.
Meanwhile, bring remaining 1 tablespoons of cornstarch and 1/3 cup water to a thick, sputtery, translucent state in a small saucepan, whisking occasionally at first and constantly as mixture thickens. Remove from heat and let cool slightly while beating whites.
Beat egg whites, cream of tarter, and remaining 1/2 teaspoon of vanilla until frothy. Gradually beat in remaining 1/2 cup of sugar until incorporated and mixture forms soft peaks. Dollop in warm cornstarch mixture, 1 tablespoon at a time; continue to beat meringue to stiff peaks.
Pour warm chocolate filling into pie shell. Using a rubber spatula immediately distribute and then spread meringue evenly over pie, making sure it attaches to piecrust to prevent shrinking. Use a spoon to create peaks all over meringue. Bake pie until meringue is golden brown, about 20 minutes. Transfer to a wire rack and cool to room temperature. Serve or refrigerate until ready to serve.