2 1/2 cups all-purpose flour
2 tsp cream of tartar
1 tsp. baking soda
1/2 tsp. salt
8 tablespoons (1 stick) unsalted butter, softened
1/2 cup vegetable shortening
1 3/4 cup sugar, divided
2 large eggs, room temperature
1 tablespoon ground cinnamon
Adjust oven rack to middle position and heat oven to 375 degrees. Line baking sheets with parchment paper.
Combine flour, cream of tartar, baking soda and salt in bowl.
With electric mixer on medium-high speed, beat butter, shortening and 1 1/2 cups sugar until light and fluffy, about 2 minutes. Add eggs, one at a time, mixing after each until well-incorporated.
Reduce speed to low, add flour mixture gradually and mix until just combined.
Combine cinnamon and remaining (1/4 cup) sugar in bowl.
Roll dough into 1 1/2-inch balls with moistened hands, toss in cinnamon-sugar to coat, then place 2 inches apart on prepared baking sheets. Bake each sheet separately until edges are light golden brown and centers are soft and puffy, 10 to 12 minutes, rotating each sheet halfway through baking.
Cool 10 minutes, then transfer to wire rack to cool completely.