In a medium saucepan, whisk together sugar, cocoa, cornstarch, and salt.
Gradually whisk in milk until smooth.
Cook over medium heat, stirring constantly, until thickened and bubbling (8–10 minutes).
Temper the eggs by slowly adding a little hot mixture into yolks, then pour back into the saucepan.
Continue to cook for 2–3 more minutes until thick and creamy.
Remove from heat; stir in butter and vanilla.
Pour filling into the baked pie crust.
Prepare the meringue: beat egg whites and cream of tartar until soft peaks form. Gradually add sugar, beating until glossy and stiff.
Spread meringue over warm filling, sealing to the edges.
Bake at 350°F for 10–12 minutes or until golden.
Cool completely, then refrigerate for at least 4 hours before serving.