In a saucepan, whisk sugar, cocoa, cornstarch, and salt.
Gradually whisk in milk until smooth.
Stir constantly over medium heat until thick.
Beat yolks and slowly add hot mixture.
Return to pan and cook 2 more minutes.
Stir for smooth texture.
Spread into baked crust.
Beat egg whites with cream of tartar.
Whip until stiff glossy peaks form.
Spread over hot filling; bake at 350°F for 10–12 minutes.
Cool, then refrigerate 2 hours before serving.
