Pre-heat your oven to 375 degrees and prepare a baking sheet with parchment paper.
Add the flour, cream of tartar, baking soda, and salt to a bowl and whisk together to combine.
In a large bowl, add the softened butter and sugar. Beat with a hand mixer until light and fluffy. (About 2 minutes) You can also do this in a stand mixer with the whisk attachment if you have one.
Add the egg, milk, and vanilla to the butter mixture and whisk again to combine.
Using a rubber spatula, add the dry mixture to the wet mixture in a few batches mixing in-between each addition. (If you're using a stand mixer, switch to the paddle attachment for this part) The dough will be very thick. Make sure you scrape up all the flour from the bottom of the bowl and incorporate it.
To make the topping, mix together the sugar and cinnamon.
Take the dough and scoop out about 1.5 to 2 tablespoons. Press it together gently in your hands and roll into a ball. Place the ball in the cinnamon/sugar mixture and roll around to coat.
Place each dough ball on the prepared cookie sheet at least 2 inches apart.
Bake for 11-12 minutes then remove from the oven and let cool on the cookie sheet before enjoying.
Store in an air-tight container for up to 5 days (if they last that long!)