Preheat oven to 375 degrees F. Line 3 baking sheets with silicone baking mats or parchment paper.
In a medium bowl, whisk together the flour, cream of tartar, baking soda, and salt.
Using a stand mixer fitted with paddle attachment, beat the butter, shortening, and sugar together on medium speed until light and fluffy, about 3 minutes. Beat in eggs, one at a time, until incorporated, about 30 seconds, scraping down bowl as needed.
Reduce speed to low and slowly add flour mixture until combined, about 30 seconds. Give dough final stir by hand to ensure that no flour pockets remain.
Combine the sugar and cinnamon for the coating in a small bowl or shallow dish. Working with 2 tablespoons dough at a time (or using a medium cookie scoop), roll into balls, then roll in cinnamon-sugar to coat. Arrange the cookies 8 per sheet and bake the cookies, 1 sheet at a time, until the edges are just set and beginning to brown but the centers are still soft, puffy and cracked (the cookies will look raw between the cracks and seem underdone), 8 to 12 minutes. Allow the cookies to cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely. The cookies can bet stored in an airtight container for up to 5 days.
Raw cookies can be frozen for up to 1 month; bake frozen cookies in 300-degree oven for 18 to 20 minutes.