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.
170 grams unsalted butter
Beat in sugar - Once the butter has cooled down, add brown sugar and beat with the paddle attachment for at least 2 minutes until lightened in color.
250 grams brown 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, 10 grams pure vanilla extract, 80 grams sourdough starter discard
Add dry ingredients - Sift the flour, cinnamon and baking soda 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.
135 grams all-purpose flour, 3 grams baking soda, 2 grams cinnamon, 4 grams kosher salt
Add mix-ins - Add the oats and chocolate chips, stir to distribute evenly. Stop the mixer and use a spatula to scrape the side and bottom of the bowl to ensure good distribution of everything throughout the cookie dough.
100 grams old-fashioned rolled oats, 174 grams bittersweet chocolate chip
Scoop and chill - Portion the cookie dough with a 3-tablespoon cookie scoop and place them on a lined baking sheet. Cover and refrigerate for at least 1 hour, and up to 3 days, before baking. Chilling the dough is essential to allow the butter to re-solidify, the flour to hydrate and the flavor to develop.
Preheat the oven to 350°F.
Bake - Place 6 dough balls on a baking sheet, and bake 1 batch at a time for about 14 minutes, until the cookies start to turn golden but the middle is still slightly underbaked.
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.