In a large bowl, cream butter with brown and granulated sugars until fluffy. Beat in egg, then vanilla. In medium bowl, sift together flour, baking soda and cream of tartar. Gradually add to butter mixture until well mixed. Divide dough in half; form two flat disks. Wrap eeach disk in plastic wrap; chill at least 3 hours. On lightly floured surface, working with one disk at a time, roll to 1/4" thickness or less. Using cookie cutters, cut out shapes. Transfer to lightly greased baking sheets, placing 1 1/2" inches appart. Bake in preheated 350F oven for about 8 minutes or until lightly browned around edges. Cool on racks. Reroll scraps of dough to form more cookies. When cool froist with Decorating Icing and decorate with sprinkles, dragees and Smarties. Makes About 40 3" Cookies