Beat butter and zest at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add sugar, beating well.
Combine flour and salt; gradually add to butter mixture, beating until blended. Shape dough into a disc.
Roll dough to 1/8-inch thickness on a lightly floured surface.
Cut with a 2 1/4- or 3 1/4-inch heart-shaped cutter; place 1/2 inch apart on parchment paper-lined baking sheets. If desired, cut 1 or 2 holes at top of each cookie (to hang or thread ribbon through after baking).
Bake at 325 for 12 to 14 minutes or until edges are lightly browned. Cool on baking sheets 5 minutes.
Transfer to wire racks; cool completely (about 20 minutes). Decorate as desired with Royal Icing and Colorful Glaze.
