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In large bowl, beat butter and shortening on high for 30 seconds.
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Add sugar, baking soda, cream of tartar and salt and beat until combined.
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Beat in egg yolks and vanilla until combined.
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Mix in flour until well combined.
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Shape into 1 inch balls and place 2 inches apart on ungreased cookie sheets.
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Bake at 300 degrees for about 20 minutes or until tops are just slightly crackled and sides are set.
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Do not let edges brown -- if they do they are over done and will not be tender.
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Transfer to wire rack to cool.
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May freeze up to 3 months.