Preheat oven to 375 degrees. line 2 baking sheets with parchment paper. Combine 1/4 cup sugar and cinnamon in shallow dish and set aside. Whisk flour, cream of tartar, baking soda, and salt together in medium bowl.

Beat butter, shortening, and remaining 1 1/2 cups sugar together on medium speed until light and fluffy, 3-6 minutes. Beat in eggs, one at a time, until incorporated, about 30 seconds, scraping down bowl as needed.

Reduce speed of mixer to low and slowly add flour mixture until combined, about 30 seconds. Give dough final stir to ensure that no flour pockets remain.

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.

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. The cookies should look raw between the cracks and seem underdone. Let cookies cool on baking sheet for 10 minutes, then transfer to a wire rack and let cool to room temperature. Makes about 2 dozen 3-4 inch cookies.