Preheat oven to 325 degrees.
Beat butter and zest at medium speed with a heavy-duty electric 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" thickness on a lightly floured surface. Cut with a 2-1/4 or 3-1/4" heart shaped cutter; place 1/2" 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 degrees 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.
To make Royal Icing---
Beat first 3 ingredients at high speed with a heavy-duty electric stand mixer, using whisk attachment, until glossy, stiff peaks form. Tint icing with desired amount of food coloring paste, and beat until blended. Place a damp cloth directly on surface of icing (to prevent a crust from forming) while icing cookies.