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In a large mixing bowl beat butter and shortening with an electric mixer on medium to high speed for 30 seconds.
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Add sugar, baking soda, cream of tartar and salt.
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Beat until combined, scraping sides of bowl occasionally.
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Beat in egg yolks and vanilla until combined.
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Beat in as much flour as you can.
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Stir in remaining flour.
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Shape dough into 1" balls.
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Place 2" apart on an ungreased cookie sheet.
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Bake in a 300 degree oven about 15 minutes or until sides are set (do not let edges brown).
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Cool on cookie sheet for 1 minute.
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Transfer to a wire rack and let cool.
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Enjoy.