Cream together butter, margarine and powdered sugar. Beat in egg and vanilla. Combine flour, soda, cream of tartar and salt and add to creamed mixture. Mix well. Divide dough in half. Form each half into a 2-inch round roll. Wrap in waxed paper or aluminum foil. Chill in refrigerator or freezer. Slice 1/4-inch thick. Bake on ungreased cookie sheet in 350-degree oven 8 to 10 min. Cool on rack. Makes about 5 doz. cookies. Reprinted with permission from: Powdered Sugar Cookies from the C and H Sugar Kitchen by Jean Porter Electronic format by Karen Mintzias File ftp://ftp.idiscover.co.uk/pub/food/mealmaster/recipes/c_hsugar.zip