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Preheat oven to 350F
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Cream butter and sugar for 4-5 min until light and fluffy. Add egg and vanilla, then cream for 1-2 minutes longer
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Stir in flour, cream of tartar, baking soda, and salt, just until combined
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In small bowl, mix remaining 1/8 cup sugar and cinnamon
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Wrap dough and refrigerate for 20-30 minutes (optional)
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Roll dough into smooth balls and coat well with cinnamon mixture
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Place balls on parchment paper-lined baking sheet and press down in the center of the ball with your thumb
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Bake for 9-11 minutes, until the edges just begin to turn a light golden color. Let cool on the baking sheet for a few minutes and then transfer to wire rack