In medium bowl, combine flour, cornstarch, baking powder and salt.
In large bowl, beat butter and sugar with electric mixer on medium speed until light and fluffy. Beat in egg, lemon zest, and lemon juice. Add flour mixture; mix well. Drop by rounded tablespoons onto cookie sheet and flatten slightly with palm of hand.
Bake 13-15 minutes or until light golden brown. Cool on cookie sheet 5 minutes. Remove to cooling grid; cool completely.
To prepare glaze, in small bowl, whisk together confectioners’ sugar and lemon juice. Spoon over cooled cookies.