Preheat oven to 350°. Cream butter, confectioners' sugar and salt until light and fluffy. Beat in extract. Gradually beat in flour. Shape teaspoonfuls of dough into balls (if necessary, refrigerate dough, covered, until firm enough to shape). Place 1 in. apart on ungreased baking sheets; flatten slightly with bottom of a glass dipped in granulated sugar. Bake until light brown, 10-12 minutes. Remove cookies from pans to wire racks to cool completely.
For filling, whisk together egg, sugar, lemon zest and lemon juice in a small heavy saucepan over medium-low heat until blended. Add butter; cook over medium heat, whisking constantly, until thickened and a thermometer reads at least 170°, about 20 minutes. Remove from heat immediately (do not allow to boil). Transfer to a small bowl; cool. Press plastic wrap onto surface of filling. Refrigerate until cold, about 1 hour.
To serve, spread lemon filling on half of cookies; cover with remaining cookies. If desired, dust with confectioners' sugar. Store leftovers in refrigerator. Freeze option: Freeze unfilled, undecorated cookies in freezer containers. To use, thaw cookies in covered containers. Fill and decorate as directed.