Beat butter at medium speed with an electric mixer until creamy. Gradually add 2/3 cup sugar, beating well. Add egg yolk and vanilla extract; beat 1 minute.
Combine flour and salt; gradually add to butter mixture, beating at low speed just until blended. Beat at medium speed 2 minutes.
Divide dough into 3 portions; wrap individually in plastic wrap, and chill 8 hours.
Line 3 baking sheets with parchment paper. Sprinkle each lightly with flour.
Unwrap 1 dough portion, and roll on a lightly floured surface to 1/4-inch thickness. Cut into heart shapes, using a 2-inch cookie cutter. Place on prepared baking sheet.
Whisk together egg and 1 tablespoon water; brush over cookies. Sprinkle with sparkling sugar. Repeat procedure with remaining dough, rerolling scraps, if desired.
Bake at 350° for 8 to 10 minutes or until lightly browned; cool on baking sheet 10 minutes. Transfer to wire racks to cool completely. Freeze up to 3 weeks, if desired. Serve with your favorite ice cream or sherbet, if desired.
