Heat oven to 350°F. Combine all shortbread ingredients minus the flour in large bowl. Beat at medium speed, scraping bowl often, until creamy. Add flour; beat at low speed until mixture resembles coarse crumbs.
Press dough evenly into lightly greased jelly-roll pan. Bake 30-35 minutes or until light golden brown. Cool completely.
Combine all glaze ingredients in small bowl with wire whisk until smooth.
Spread thin layer of glaze over cooled shortbread; sprinkle with 1 tablespoon lemon zest, if desired. Let stand 30 minutes to set. Cut into strips.