COMBINE shortening, granulated sugar and brown sugar in large bowl; beat at medium speed until well blended. Add milk. Beat in eggs, one at a time. Add vanilla.
COMBINE flour, baking soda, cream of tartar and salt in separate large bowl. Mix into shortening mixture until well blended. Chill 1 hour.
HEAT oven to 350°F.
ROLL out 1 ⁄ 3 of dough at a time to about 1⁄4-inch thickness on floured surface. Cut out with cookie cutters. Place 2 inches apart on ungreased baking sheets. Sprinkle with colored sugars and decors or leave plain and frost when cooled.
BAKE 6 to 8 minutes or until edges of cookies are slightly golden. Remove immediately to cooling rack.
BUTTERY CREAM FROSTING:
COMBINE powdered sugar, shortening and vanilla in bowl of electric mixer; slowly blend in milk to reach desired consistency.
BEAT on high speed for 5 minutes or until smooth and creamy. Tint frosting with food coloring, if desired.
Spread a thin layer of frosting on cookies and decorate with sparkling sugars or sprinkles while frosting is still wet to ensure the decorations stay in place. If you're planning to pipe on frosting decorations, make sure to allow the base coat of frosting, and then each addition layer, to dry completely before adding another.