Melt the butter in a small saucepan over medium heat. Continue to cook, swirling the pan occasionally. The butter will foam and crack and pop. Keep swirling the pan until the butter develops a nutty aroma and brown bits form at the bottom.

Once the bits are amber in color, 2 to 3 minutes, remove from heat and pour into a heatproof mixing bowl. Let butter cool to room, about 1 hour.

Preheat the oven to 375°F. Line 2 baking sheets with parchment paper; set aside.

In a small bowl whisk the flour, cream of tartar, baking soda, and salt together.

In the bowl of a stand mixer fitted with the paddle attachment, or with a handheld electric mixer, beat the butter and both sugars on medium speed for 3 minutes, until fluffy and pale in color. Stop to scrape down the sides and bottom of the bowl

Add the egg, followed by the egg yolk and vanilla, beating for 20 seconds after adding each. Scrape down the bottom and sides of the bowl.

With the mixer on low speed, slowly add the flour mixture and mix until just combined.

Combine the sugar and cinnamon in a small bowl.

Roll the dough into 1.5-tablespoon-sized balls. Dip each ball in the cinnamon-sugar mixture, rolling it around to coat.

Place the cookie balls 2-inches apart on the prepared baking sheets. Bake 1 sheet at a time for 10-12 minute, until the edges are just beginning to brown but the centers are puffy and soft. (The centers will look underdone, this is okay.)

Cool the cookies on the pan for 10 minutes then transfer to a wire rack to cool completely.