Preheat oven to 375. Line baking sheets with parchment paper.
2
Combine 1/4c sugar and cinnamon in dish and set aside.
3
Whisk flour, cream of tartar, baking soda, and salt together in medium bowl.
4
Beat butter, shortening, and 1 1/2 cups sugar together on medium speed until light and fluffy, 3-6 minutes.
5
Beat in eggs, one at a time, until incorporated, about 30 seconds, scraping sides of bowl as needed.
6
Reduce speed of mixer to low and slowly add flour mixture until combined, about 30 seconds. Give dough final stir to ensure no flour pockets remain.
7
Working with 2 tablespoons of dough at a time, roll into balls. Working in batches, roll dough balls in cinnamon sugar mixture to coat and set on prepared baking sheet spaced 2 inches apart.
8
Bake 1 sheet at a time until edges of cookies are set and just barely beginning to brown, but centers are still soft and puffy, about 10-12 minutes. Let cookies cool on baking sheet for 10 minutes, then transfer to wire rack and let cool to room temperature.
