Gather all ingredients. Preheat the oven to 350 degrees F (175 degrees C). Separate eggs. Set whites aside for meringue.

Make the pie: Mix sugar, cocoa, and cornstarch together in a medium saucepan. Gradually mix in milk. Place over medium-high heat and cook, stirring often, until thickened and bubbly, 4 to 5 minutes. Reduce the heat to medium-low; cook and stir for 2 minutes. Remove from the heat.

Pour about 1 cup hot filling into egg yolks, whisking quickly to avoid scrambling eggs. Pour mixture back into the saucepan; whisk to combine with hot filling.

Set over medium heat and bring to a gentle boil; cook and stir for 2 minutes. Remove from the heat and stir in vanilla. Pour hot filling into crust.

Make the meringue: Beat reserved egg whites and cream of tartar in a glass, metal, or ceramic bowl until soft peaks form.

Gradually beat in sugar, then continue to beat until stiff peaks form.

Spread evenly over hot filling, all the way to the crust.

Bake in the preheated oven until meringue is golden, 12 to 15 minutes.