Place the water,cornstarch and cocoa in a heavy duty medium saucepan. Whisk the ingredients together until the mixture is without lumps and smooth. Whisk in the sweetened condensed milk and slightly beaten egg yolks. Cook on low heat until mixture thickens. Add the butter and vanilla to the pan, remove from the heat and whisk until the butter and vanilla are incorporated. Pour the filling into the baked pie crust, place a sheet of plastic on top and refrigerate until completely cooled. Top with Cool Whip or whipping cream.