1In large bowl, beat butter, 1 1/2 cups powdered sugar, the vanilla and egg with electric mixer on medium speed, or mix with spoon. Stir in flour, baking soda and cream of tartar. Cover; refrigerate 2 to 3 hours or until chilled.
2Heat oven to 375°F. Divide dough in half. On lightly floured cloth-covered surface, roll each half 1/4 inch thick. Cut with 3 1/2-inch heart-shaped cookie cutter. On ungreased cookie sheet, place cutouts 2 inches apart. If desired, sprinkle with colored sugars.
3Bake 8 to 10 minutes or until delicately golden. Remove from cookie sheet to cooling rack. Cool completely, about 30 minutes.
4In small bowl, mix glaze ingredients until smooth and spreadable. Spread glaze over cookies. Decorate with decorator icings.