Preheat oven to 375 degrees.  Line a baking sheet with parchment and set aside.

1. Melt/soften your butter in a heat-safe mixing bowl.  Note: It doesn’t need to be ALL the way.  See photos for an example- I’ve found the best stage is when about half the butter is liquid, but the rest is super soft.  Like the texture of pudding.  You want it to easily stir completely smooth when you touch it.  If you butter is all the way melted, no worries!  

2. Beat in white sugar and egg yolk.

3.  Measure flour by spooning it lightly into the measuring cup and then leveling with a knife.  Sprinkle flour, cream of tartar, baking soda, and salt in the bowl over the butter mixture and mix until it forms a cohesive dough. *At this point I like to pop the bowl of dough in the freezer for 10 minutes.  It’s a great time to wipe down your counters and put ingredients away! 

4. Form one single ball of dough and set it back in the mixing bowl off to one side.  Sprinkle the 1 tablespoon of additional sugar and 1/4 teaspoon cinnamon to the other side and mix them together. Roll your dough ball around in the cinnamon-sugar mixture until it’s coated.

5. Place dough ball on your parchment-lined baking sheet and gently press down with your hand until it’s about 3/4-inch thick.  If you have extra cinnamon-sugar left in your bowl, you can sprinkle more on top!

6. Baking Note: All ovens are different!  My snickerdoodle is done after about 11 1/2 minutes. I would set a timer for 10 minutes and watch it closely after that.  It should take anywhere from 10-13-ish minutes.  I’ve found the best time to take it out of the oven is when you see those signature crackly edges around about the outside inch of the cookie.  As it cools it will continue cooking for a few minutes and the rest of those crackles will appear!  This produces a cookie with crisp edges and keeps the center soft.

Cool for at least 15 minutes before using an extra large spatula to transfer cookie to cooling rack.