In a medium bowl, whisk together the flour, cream of tartar, baking soda, salt, and chai spice blend. Set aside.
In a large bowl or stand mixer, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy (about 3-4 minutes).
Beat in the eggs one at a time, then add the vanilla extract. Mix until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Don't overmix!
Cover the dough and chill in the refrigerator for at least 1 hour (or overnight for even better results).
Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
Roll the chilled dough into 1-inch balls, then roll each ball in the cinnamon-sugar mixture to coat.
Place the coated dough balls on the prepared baking sheets, about 2 inches apart.
Bake for 10-12 minutes, or until the edges are lightly golden and the centers are set but still soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.