Grab another bowl and whisk together the flour, baking soda, cream of tartar, salt, and cinnamon. In a larger bowl, cream the unsalted butter and sugar at high mixer speed until smooth. Then, toss in the egg, yolk, and a little vanilla extract and mix it all up at medium-high until combined. Scrape the bowl as you go.
With your mixer whirling at low speed, gradually add our dry ingredients to wet in three big batches. The dough will be pretty thick.
Roll the dough into small balls. Aim for around 1 ½ tablespoons of dough for each ball. Give them a good coat in the cinnamon-sugar mix and space them on your baking sheets (3 to 3.5 inches apart).
Pop them in the oven for about 10 minutes — they'll come out all soft and puffy. While they're still warm, gently press them down with a fork or spoon to flatten their tops.
Let them cool on their baking sheets for 8 to 10 minutes, then transfer them to a rack to chill completely. Enjoy!