Prepare the Chocolate Filling: In a medium saucepan, whisk together the cocoa, cornstarch, egg yolks, sugar, salt, and vanilla. Gradually add the milk while whisking constantly to blend the ingredients smoothly. Using a whisk helps prevent lumps and ensures a creamy filling.
Cook the Filling: Place the saucepan over medium-high heat, stirring continuously as the mixture heats. The filling will begin to thicken after about 8–10 minutes. Keep whisking until it reaches a smooth, pudding-like consistency that coats the back of a spoon. Once thickened, remove from heat immediately to prevent scorching.
Fill the Pie Shell: Pour the hot chocolate filling into your pre-baked pie shell, spreading it evenly. The filling should still be hot when you top it with meringue — the heat helps cook the underside of the meringue and creates a perfect seal.
Prepare the Meringue: In a clean mixing bowl, beat the egg whites with an electric mixer on medium speed until foamy. Add the cream of tartar and continue beating until soft peaks form. Gradually add the powdered sugar, one tablespoon at a time, beating until the meringue becomes glossy and holds firm peaks.
Top the Pie: Spoon the meringue over the warm chocolate filling, spreading it all the way to the edges of the crust to create a seal. Use the back of a spoon to make decorative swirls or peaks on top — these will brown beautifully in the oven.
Bake the Meringue: Place the pie in a 425°F (220°C) oven for 4–5 minutes, or until the peaks of the meringue are lightly browned. Watch it closely — meringue can go from golden to burnt very quickly!
