Combine powdered sugar and butter in large bowl. Beat at medium speed, scraping bowl often, until well mixed. Add 2 egg yolks and vanilla; continue beating until well mixed. Add flour and salt; beat at low speed until well mixed.
Divide dough in half. Shape each into a ball; flatten to about 1/2 inch. Wrap each in plastic food wrap; refrigerate 1 hour or until firm.
Meanwhile, combine 1 egg yolk and water in small bowl until well mixed. Divide mixture into several small cups; tint each with food color as desired. Cover; set aside.
Heat oven to 375°F. Roll out dough, one-half at a time (keeping remaining dough refrigerated), on lightly floured surface to 1/8-inch thickness. Cut with 3-inch heart-shaped cookie cutter. Place 2 inches apart onto ungreased cookie sheets.
Using small clean paintbrushes, paint thin layer of egg yolk paint on cookies to form desired designs. Bake 5-8 minutes or until edges are lightly browned. Remove from cookie sheets. Cool completely.
Variation: Frosted Heart Cookies: Omit preparation of egg yolk paint and painting of cookies before baking. Bake cookies as directed. Frost cooled cookies with Powdered Sugar Glaze. Let stand on cooling rack 6 hours or until dry. To paint, place small amounts of paste color on a piece of waxed paper or small amounts of liquid food color in small cups. If using paste colors, dip small brush in water and then in paste color. Paint designs onto frosted cookies, painting largest part of design first. When using more than one color on a cookie, let each color dry before adding another color.
If egg yolk paint thickens while rolling out dough, stir in a few drops water.