Brown the butter - Heat unsalted butter in a light colored saucepan over medium heat. Once the butter melts, it will become foamy. When the foam subsides, you will see clear bubbles, start whisking continuously at this point to keep the milk solids from burning. When the butter is caramel in color and smells nutty - about 8 to 9 minutes - take it off the heat and immediately pour it into the stand mixer bowl and allow to cool.
113 grams unsalted butter
Beat in sugar - Once the butter has cooled down, add granulated sugar and beat with the paddle attachment for at least 2 minutes until lightened in color.
150 grams granulated sugar
Incorporate sourdough starter - Add the egg yolk, vanilla extract, sourdough starter and continue beating for at least another 2 minutes until fluffy and creamy.
1 large egg yolk, 5 grams pure vanilla extract, 75 grams sourdough starter discard
Add dry ingredients - Sift the flour, cinnamon, baking soda and cream of tartar into a small mixing bowl, stir in the kosher salt until evenly distributed. Add the dry ingredient mixture into the wet ingredient mixture in the stand mixer bowl and stir until incorporated. Stop the mixer and scrape down the sides and bottom of the bowl as necessary.
180 grams all-purpose flour, 3 grams cinnamon, 3 grams baking soda, 4 grams cream of tartar, 2 grams kosher salt
Scoop and chill - Portion the cookie dough with a 1 ½-tablespoon cookie scoop and place them on a lined baking sheet. Roll into balls. Cover and refrigerate for 1 hour before baking. Chilling the dough allows the brown butter to re-solidify, the flour to hydrate and the flavor to develop.
Preheat the oven to 350°F.
Prepare the cinnamon sugar topping - Mix together granulated sugar and cinnamon in a shallow bowl.
25 grams granulated sugar, 3 grams cinnamon
Bake - Roll the dough balls in cinnamon sugar, place 12 on a half sheet baking sheet, and bake for about 13 minutes, until the edges of the cookies start to turn lightly golden.
Cool - Remove the baking sheet from the oven, and allow the cookies to sit on the baking sheet to finish baking from the residual heat for 10-15 minutes before removing them to a wire rack to cool completely.