Preheat oven to 350°F (175°C). 2
In a saucepan, melt butter over medium heat.
Whisk together sugar, flour, and cocoa powder, then gradually mix into melted butter.
Slowly add milk, stirring continuously, and cook until thickened (5-7 minutes).
Temper egg yolks by adding a spoonful of the hot mixture, then return the yolks to the saucepan.
Cook for another 2-3 minutes, stir in vanilla, and remove from heat.
Pour chocolate filling into the pre-baked crust.
Beat egg whites with cream of tartar until soft peaks form, then gradually add sugar and beat to stiff peaks.
Spread meringue over the pie, sealing edges, and bake for 10-12 minutes until golden brown.
Cool completely before serving.
