1. Preheat oven to 325°. Beat butter and zest at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add sugar, beating well.
2. Combine flour and salt; gradually add to butter mixture, beating until blended. Shape dough into a disc.
3. 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).
4. 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.