Cream butter and sugar until light and fluffy.
Beat in egg and vanilla.
Combine flour, baking soda and salt; blend into butter mixture.
Divide dough into thirds, shape into thick rectangles and place in gallon-sized plastic food storage bags, roll to ⅛-inch thickness then chill dough until firm.
Preheat oven to 375°F.
Cut bag at seams and uncover top of dough. Cut dough with 2-inch heart-shaped cutter. Dough scrapes can be placed within the cut bag for re-rolling (chill if necessary.)
Use an off-set spatula to transfer cut-outs to cookie sheet. Allow about ½-inch between shapes on unbuttered cookie sheet. (Cookies can be sprinkled with colored sugars before baking or left plain to ice and sprinkle with colored sugar after baking.)
Bake 7 to 8 minutes or until edges are slightly browned.
Allow cookies to cool.
Spread with icing and sprinkle with colored sugars if desired.
Store in air tight container.
Blend in powdered sugar.
Beat in vanilla and milk (or water). Blend in food color if desired. Add additional sugar or liquid if necessary for desired spreading consistency.