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1
Adjust an oven rack to the middle position and heat the oven to 375 degrees.
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2
Combine 1/4 cup of the sugar and the cinnamon in a shallow dish for coating and set aside.
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3
Whisk the flour, cream of tartar, baking soda, and salt together in a large bowl and set aside.
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4
Beat the butter, shortening, and remaining 1 1/2 cups sugar together in a large bowl using an electric mixer on medium speed until light and fluffy, 3 to 6 minutes.
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5
Beat in the eggs, one at a time, until incorporated, about 30 seconds, scraping down the bowl and beaters as needed.
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6
Reduce the mixer speed to low and slowly mix in the flour mixture until combined, about 30 seconds.
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7
Give the dough a final stir with a rubber spatula to make sure it is combined.
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8
Using wet hands, roll 2 tablespoons of dough at a time into balls, then roll in the cinnamon sugar to coat and lay on two parchment-lined baking sheets, spaced about 2 1/2 inches apart.
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9
Bake the cookies, one sheet at a time, until the edges are set and just beginning to brown but the centers are still soft and puffy, 10 to 12 minutes, rotating the baking sheet halfway through baking.
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10
Let the cookies cool on the baking sheet for 10 minutes, then serve warm or transfer to a wire rack and let cool completely.
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11
To Make Ahead: The dough can be made through step 3 and either covered tightly with plastic wrap and refrigerated for up to 3 days or frozen.
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12
To freeze the cookie dough, portion into individual balls, roll in the cinnamon and sugar, and freeze on a parchment- or wax-paper-lined plate (don't let them touch or they will fuse together) until completely firm, 2 to 3 hours.
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13
When ready to bake, reduce the oven temperature to 300 degrees and increase the baking time to 17 to 22 minutes.
