Make the pie filling by placing a medium saucepan over low heat, and combining sugar, cocoa powder, all-purpose flour, and salt. Stir ingredients until fully combined.
Add milk and egg yolks, and whisk until all ingredients are combined. Continue to cook, stir whisking continually, until the mixture begins to thicken.
Remove from heat, and add butter and vanilla. Stir to combine. Cover with plastic wrap directly on the chocolate filling, and allow to cool.
Make the meringue while your pie filling is cooling. In a mixing bowl, add egg whites, and whisk on high speed until stiff peaks form. Add cream of tarter and continue to whisk on high.
Add sugar and vanilla extract and stir just to combine.
Preheat oven to 350 °F
Spoon the meringue over the entire surface of the pie. You can use a rubber spatula to create peaks, then place the pie into the oven and bake for 10 to 12 minutes, or until the meringue starts to turn a golden brown.