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1
Line two baking sheets with silicone baking mats or parchment paper. Set aside.
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2
Place butter, granulated sugar, and brown sugar in a medium bowl. Use hand mixer to beat on medium speed until well-combined, about 1 minute.
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3
Add egg, egg yolk, and vanilla extract. Continue to beat until smooth, about 30 seconds.
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4
Add flour, cornstarch, baking soda, cream of tartar, and salt to the bowl. Beat on low speed, scraping sides of bowl with a rubber spatula as needed, until a smooth dough forms.
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5
Divide cookie dough into 30 balls, using roughly 1 1/2 tablespoons of dough for each, and arrange them 1 1/2 inches apart on baking sheets. Transfer sheets to freezer for 10-15 minutes.
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6
Preheat oven to 350u00b0F.
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7
Place sugar, cinnamon, and salt in a mixing bowl. Whisk to combine. Remove baking sheets from freezer. Roll each cookie in cinnamon sugar, creating a thick coating, and return to baking sheet.
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8
Transfer baking sheets to middle rack of oven.
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9
Bake cookies until puffy and golden brown at edges, 15-17 minutes.
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10
Check to see that cookies are done. Remove from oven or add time as needed.
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11
Allow cookies to rest on sheets for 5-10 minutes, then transfer to a wire rack to cool completely before serving.
