Brown the Butter: In a light-colored saucepan over medium heat, melt butter. Swirl frequently as it foams and crackles. After 5 to 6 minutes, dark amber milk solids will settle to the bottom and release a rich, nutty hazelnut aroma. Immediately pour brown butter into a heatproof bowl (scraping all browned specks) and let cool to room temperature (about 25 minutes).

Whisk Dry Ingredients: In a medium mixing bowl, whisk together all-purpose flour, cream of tartar, baking soda, and fine sea salt until fully blended. Set aside.

Beat Wet Ingredients: Add brown sugar and white sugar to the cooled brown butter. Beat with a hand mixer or wooden spoon for 2 minutes until glossy. Add the egg, egg yolk, vanilla extract, and milk. Beat for 1 minute until smooth, creamy, and slightly pale.

Combine Dough & Chill: Pour dry ingredients into wet ingredients. Fold with a silicone spatula just until combined and no dry flour pockets remain (do not overmix). Cover bowl and chill in the refrigerator for 30 minutes to firm up the butter and intensify the nutty flavor.

Roll in Cinnamon Sugar: Preheat oven to 350°F (175°C) and line two large baking sheets with parchment paper. In a small bowl, stir together the granulated sugar and ground cinnamon. Use a 2-tablespoon cookie scoop to form dough balls, then roll each ball generously in the cinnamon sugar mixture until thickly coated.

Bake to Crinkly Perfection: Place dough balls 2 inches apart on prepared baking sheets. Bake at 350°F for 9 to 11 minutes, until edges are set and tops are puffed with delicate crinkles, while centers remain soft and slightly underbaked. Remove from oven and let cool on pan for 10 minutes before transferring to a wire cooling rack. Enjoy warm with a glass of milk!