In large bowl, break up cookie dough. Stir or knead in flour until well blended. Reshape into log. If too soft to cut into slices, refrigerate up to 30 minutes.
Heat oven to 350°F. In small bowl, beat egg yolk. Stir in food color until well blended.
Cut dough into 32 (1/4-inch) slices. On ungreased cookie sheets, place slices 2 inches apart. Dip thumb or index finger into egg mixture to generously coat with color. Press finger onto each slice of dough at an angle twice to make heart shape. Sprinkle with sugar.
Bake 7 to 11 minutes or until edges are light golden brown. Cool 1 minute; remove from cookie sheets to cooling racks. Cool completely, about 5 minutes. With decorating icing, write names or messages on cookies around heart.