Cream butter and sugars and then mix in the oil until well blended. Add eggs and vanilla, then mix. Sift 2 cups of flour with soda and cream of tartar, then add to egg mixture. Stir in the remaining 2 cups of flour. Mix well. Refrigerate 4 hours or more(or 1 hour in the freezer). When ready to bake, shape dough into walnut-sized balls. Place on a well-greased cookie sheet 4 inches apart. Flatten dough using a flat bottomed damp glass dipped in sugar. Bake in a pre-heated 325 degree oven for 12-15 minutes or until delicately browned.
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