Bake your pie shell as directed. Then set to the side
Mix together water,corn starch and cocoa until smooth.
Then stir in condensed milk and egg yolks, Mix till its smooth.
Cook in microwave until thick. I cook for 2 minutes then stir it good then put it on another 2 minutes.Stir in butter and vanilla. Then its ready to pour into baked pie shell. You can go a extra min in microwave if you like a thicker filling.
You can make a meringue or just top with cool whip.