Mix sugar and corn starch in saucepan. Gradually stir in water. Cook over medium heat, stirring constantly, until mixture thickens and boils. Boil one minute. Slowly stir at least half the hot mixture into egg yolks. Then blend into hot mixture in saucepan. Boil one minute longer, stirring constantly. Remove from heat. Continue stirring until smooth. Blend in butter, lemon juice and rind. Pour into baked pie shell. Cover with meringue and bake. Serve as soon as cool.
Heat oven to 400 degrees. Beat egg whites with cream of tartar until frothy. Gradually beat in sugar, a little at a time. Continue beating until stiff and glossy. Do not underbeat. Beat until sugar is dissolved. Beat in flavoring. Pile meringue onto hot pie filling, being careful to seal the meringue onto edge of crust to prevent shrinking or weeping. Swirl or pull up points for decorative top. Bake 8 to 10 min., or until delicately browned. Cool away from drafts.