Combine flour and salt in small bowl. Beat butter, sugar, lemon peel, and vanilla extract in large mixer bowl until fluffy. Add flour mixture just until blended. Stir in 1 cup morsels. Divide dough into 4 even pieces. Gently shape each piece into an 8 x 1-inch diameter log. Wrap logs tightly in wax paper. Refrigerate for at least 1 hour or until firm. Preheat oven to 350 degrees F. Unwrap logs; cut each log with serrated knife into 16 slices, about 1/2-inch thick. If necessary, reshape edges of cookies. Place 2 inches apart onto ungreased baking sheets. Bake for 13 to 14 minutes or until edges are set. Cool for 2 minutes; remove to wire racks to cool completely. Line baking sheets with wax paper. Microwave remaining 1 cup morsels and vegetable shortening in small, uncovered, microwave-safe bowl on medium-high (70%) power for 1 minute; stir. Morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until morsels are melted. Dip top half of cookies into melted morsels. Scrape off excess against side of bowl. Place on prepared baking sheets. Refrigerate for 20 minutes or until set. Makes about 5 dozen cookies.
Note: Dough logs may be frozen up to 1 month in advance. Place wrapped logs in plastic freezer storage bags.