Pre-bake your pie crust until golden and set aside to cool.
In a saucepan, whisk together 1 cup sugar, flour, and cocoa powder. Add milk gradually.
Heat over medium, stirring constantly, until thick.
In a bowl, beat egg yolks. Temper with hot filling, then return to saucepan. Cook 1–2 minutes more.
Remove from heat, stir in butter and vanilla, then pour into crust.
In another bowl, beat egg whites and cream of tartar. Gradually add sugar and beat to stiff peaks.
Spoon meringue over hot filling and seal to crust edges.
Bake at 350°F for 8–10 minutes until golden. Cool at room temp, then chill if desired.