Cream Butter And Sugar Until Light And Fluffy. Beat In Eggs. Then Stir In Powdered Sugar, Vanilla, Cream Of Tartar And Salt. Alternately Add Oil And Flour Beating Until Smooth And Fluffy. Refrigerate For 2 Hours. Heat Oven To 350 F. Roll Rounded Tablespoonfuls Of Dough Into Balls. Place Two Inches Apart On A Lightly Greased Cookie Sheet. Flatten Each Cookie With The Bottom Of A Glass That Has Been Lightly Oiled And Sugared. Bake For 8 To 10 Minutes. Cool On Wire Rack.