In a large bowl, add the flour, baking soda, cinnamon, cream of tartar and salt. Whisk to combine and set aside.
In the bowl of a stand mixer, add the butter, brown sugar and granulated sugar. Mix on medium speed for 2-3 minutes, until very light and fluffy. Scrape the sides and bottom of the bowl. Add in milk and vanilla and mix again on medium speed for 2 minutes.
With the mixer on low, add the flour a little at a time, mixing just until incorporated. Do not overmix.
Prepare a baking sheet with a sheet or parchment paper. Using a large cookie scoop, scoop the dough onto the prepared baking sheet. You can place the cookies close together, as you’ll be chilling them before baking and this will save room in your fridge. Cover the cookie dough with plastic wrap and chill overnight in the fridge.
When ready to bake, preheat the oven to 350ºF (177ºC). Line a second baking sheet with parchment paper and set aside. In a small, shallow bowl mix together the granulated sugar and cinnamon. Remove the cookie dough from the fridge. Roll each cookie dough ball in the cinnamon sugar mixture until evenly coated all around. Place on the prepared baking sheet, spacing the cookies out at least 2 inches apart.
Bake 12-14 minutes, rotating the baking sheet halfway through the bake time. I like to bake one tray at a time, keeping the rest of the cookies in the fridge while baking the first tray. If you are short on time, you can bake both trays at the same time. Be sure to rotate the trays top to bottom as well as front to back halfway through. The cookies may look a bit uncooked in the middle. That’s okay, as they’ll continue cooking on the hot trays after you remove them from the oven. Allow cookies to cool on the baking sheets for at least 15 minutes before moving to cooling racks.