In a bowl, cream butter and confectioners' sugar. Beat in extract. Combine flour and cornstarch; beat into creamed mixture. Divide into two balls; wrap in plastic and refrigerate for 1 hour.
On a lightly floured surface, roll each portion of dough to 1/8-in. thickness. Cut into 2-in. rounds. Place on ungreased baking sheets. Bake at 350° for 10-12 minutes or until edges are lightly browned. Remove to wire racks to cool.
For filling, in a small bowl, cream butter and confectioners' sugar. Beat in lemon juice and zest. Spread over the bottoms of half of the cookies; top with remaining cookies.