Blend cornstarch and cold water in saucepan. Add boiling water. Cook, stirring until clear and thickened. Let stand until cold. With electric beater on high speed, beat egg whites until foamy. Add cream of tartar. Continue to beat until they start to turn white. Gradually, add sugar; beat until stiff but not dry. Turn mixer to low speed; add soda and vanilla. Gradually, beat in cold cornstarch mixture. Raise mixer again to high speed; beat well. Spread meringue over cooled pie filling. Bake @ 350 degrees for at least 10 minutes.