Blend sugar, flour, and cocoa together in a 4 quart pan.
Add milk. Stir with a spoon until well blended.(I like to use a wooden spoon.)
Cook over medium heat, stirring constantly, until the mixture is hot.
Beat egg yolks in a seperate bowl and add about 1/2 cup of cocoa mixture to it to temper the eggs. Mix and return to pan and keep stirring.
When the mixture thickens and heavy coats the spoon add butter and vanilla. Mix well. Pour into baked pie shell.
Make meruinge with whites. Add 1/4 cup sugar to whites a little at a time until stiff peaks form. spread over chocolate mixture and bakein 375 degree oven for about 15 minutes or until meruinge is lightly browned. Cool to room temp and serve.