Whisk sugar, cocoa, cornstarch, and salt.
Gradually stir in milk until smooth.
Heat over medium until bubbling and thickened.
Whisk 1 cup of hot mix into yolks; return to pan.
Add butter and vanilla.
Spread into crust.
Beat whites and cream of tartar; add sugar gradually.
Cover filling and seal edges.
350°F until golden.
Cool, then refrigerate 2 hours.
