1. Preheat oven to 325°. Beat butter at medium speed with a heavy-duty electric stand mixer until creamy. Gradually add powdered sugar, beating well.
2. Combine flour and salt; gradually add to butter mixture, beating until blended.
3. Roll dough to 1/8-inch thickness on a lightly floured surface. Cut with a 2-inch snowflake-shaped cutter, and place 1 inch apart on parchment paper-lined baking sheets.
4. Bake at 325° for 11 to 13 minutes or until edges are lightly browned. Cool on baking sheets 5 minutes. Transfer to wire racks, and cool completely (about 40 minutes). Decorate with Royal Icing, and sprinkle with sparkling sugar.