Brown the Butter: Heat the butter in a pan or pot on the stove over medium heat. Swirl the butter around and stir every few minutes until little brown flecks are on the bottom of the pan and the butter smells nutty and delicious. Be careful not to overheat as it can burn the butter. Pour the brown butter along with all the little brown bits on the bottom of the pan into a medium sized bowl and let sit for 5 minutes to cool
Add the dark brown sugar and granulated sugar to the bowl with the brown butter. Stir to combine.
Mix in the egg yolk, sourdough discard and vanilla extract. Mix together with a spoon until the mixture turns light and fluffy.
Add the flour, baking soda, baking powder and salt to the middle of the bowl. Mix together using a light hand so the flour mixture is evenly dispersed throughout the dough
Add the chocolate chips and nuts and stir.
Place the whole bowl into the freezer and chill the dough for 15 minutes. I just put mine in the refrigerator overnight.
Preheat the oven to 375 degrees. Scoop the dough into balls and place onto a parchment lined baking sheet about 12 cookies per baking sheet
Bake cookies at 375 degrees until the cookies are puffed up and the edges are a little crispy for about 8-10 minutes. Let the cookies sit for about 5 minutes on the baking sheet to set up before removing
