Sift together the flour, cream of tartar, baking soda and salt and set aside. In a separate bowl, cream together the butter and sugar until fluffy. Add the eggs one at a time, beating well after each addition. Beat in the vanilla. Gradually add the dry ingredients 1/3 at a time, alternating with the milk and mix until well combined. Flatten the dough, wrap in plastic wrap and chill in the refrigerator for at least 30 minutes. Overnight is even better.
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