To brown butter, heat unsalted butter in a light colored saucepan over medium heat, whisking occasionally. The butter will melt, become foamy, and then form clear bubbles, keep whisking continuously at this point to keep the milk solids from burning. When the butter is caramel in color and smells nutty, turn off the heat.

Allow the butter to cool completely before making the cookie dough. It’s okay if the butter re-solidifies, but is still soft.

Add brown sugar to the cooled brown butter and beat with the paddle attachment for 2 minutes until lighter in color.

Add the egg, egg yolk, vanilla extract, sour cream and continue beating for another 2 minutes until fluffy.

In another mixing bowl, sift together the flour, baking soda, and baking powder. Stir in the kosher salt and give it a good mix to distribute. Add the flour mixture to the wet ingredient mixture in the stand mixer bowl, stir until incorporated. Stop the mixer and scrape down the sides and bottom of the bowl as necessary.

Add the chocolate chips and stir to incorporate. Stop the mixer and scrape down the bowl with a spatula and give it another good mix as needed.

Portion the cookie dough with a large cookie scoop and place it on a parchment lined baking sheet. Refrigerate while the oven is preheating. (Optional: If you want the cookie to be rounder, roll the portioned dough into balls.)

Preheat the oven to 350°F.

Place 6 dough balls on a baking sheet, about 2 inches apart, and bake 1 batch at a time for 14 - 15 minutes, until the outer edges of the cookies are golden brown.

Allow the cookies to cool on the baking sheet for 10 minutes before removing them to a wire rack to cool completely.