Whisk together the dry ingredients in a bowl (flour, cocoa powder, cream of tartar, baking soda, salt) and set aside.
In a stand mixer fitted with the paddle attachment (or with a hand mixer), cream together the butter, chai masala blend and sugars for 3 minutes on medium speed.
Scrape down the sides, add the egg, and mix on medium speed for another minute. Scrape down the sides once more.
With the mixer running on low speed, spoon in the dry ingredients until just combined.
Place the dough in the fridge (covered) for one hour.
Preheat the oven to 350℉/177℃ and line two large baking sheets with parchment paper. Add the granulated sugar to a bowl.
Make ~2 tablespoon sized balls (or ~40g), rolling in between your hands, then roll through the sugar.
Evenly space the cookies on the prepared baking sheets and bake for 11-13 minutes.
Remove from the oven and bang the pan on the counter a few times, then place on a wire rack to cool for a few minutes.
Transfer the cookies to a wire rack to finish cooling, then enjoy.