1. Preheat oven to 350°. Beat butter, granulated sugar, and vanilla at medium speed with an electric mixer until fluffy. Add egg, beating until blended.
2. Combine flour and salt; gradually add to butter mixture, beating just until blended.
3. Divide dough into 2 equal portions; flatten each portion into a disk. Cover and chill 10 minutes.
4. Place 1 dough disk on a heavily floured surface, and roll to 1/8-inch thickness. Cut with a 3-inch heart-shaped cutter. Place 2 inches apart on lightly greased baking sheets. Repeat procedure with remaining dough disk.
5. Bake at 350° for 10 to 12 minutes or until edges are lightly browned. Let cool on baking sheets 5 minutes; transfer to wire racks, and let cool completely (about 30 minutes).
6. Whisk together meringue powder and 2 Tbsp. water. Brush cookies with mixture, or dip cookies in mixture, and sprinkle with sanding sugar.
For jumbo cookies: Cut rolled-out dough with a 4 1/2-inch heart-shaped cutter. Bake at 350° for 15 to 17 minutes or until edges are lightly browned. Makes: about 1 dozen.
For bite-size cookies: Cut rolled-out dough with a 1-inch heart-shaped cutter. Bake at 350° for 8 to 11 minutes or until edges are lightly browned. Makes: about 20 dozen.
You may need to prepare additional meringue powder-and-water mixture as you decorate cookies. Sanding sugar may also be called "fine" sugar.