Sift together sugar, flour and cocoa. Gradually stir in evaporated milk. Pour this into a medium saucepan and heat over medium heat until mixture comes to a boil and thickens.
Reduce heat, cook and stir for about 4 minutes. Add small amounts of hot mixture to combined egg yolks, stirring well after each addition. Return egg mixture back to pan stirring the whole time.
Cook over medium heat 1 to 3 minutes, mixture will become very thick. Remove from heat and stir in butter and vanilla. Cool for about 5 minutes and then pour into pie shell. Cover with meringue and bake at 350 for 12-15 minutes.
For meringue, In a mixing bowl, combine egg whites with the cream of tartar and vanilla. Beat at high speed with an electric mixer until soft peaks form.
Gradually add sugar and continue beating at high speed until stiff peaks form and sugar is dissolved. Cover pie with meringue and bake to lightly brown.