In a mixing bowl, cream butter, oil, and sugars. Add egg and vanilla. Combine flour, baking soda and cream of tarter; gradually add to the creamed mixture. Chill for several hours or overnight. Roll into balls. Roll in the sugar. Place on baking sheet. Flatten with fork criss crossing with the fork. Bake at 350 degrees for 13-15 minutes. Or until lightly browned. Be careful not to over cook. Cool on wire racks.
note: you may also drop by rounded teaspoonfuls and flatten with fork then just sprinkle with sugar.