Brown 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 chocolate chunks (or chips) and stir into the dough.
Place the whole bowl into the freezer and chill the dough for 15 minutes. It is possible to bake these cookies right away, but they will not be quite as puffy and will spread a lot more than the chilled dough. You can also chill the dough in the refrigerator for up to 24 hours or freeze the dough in small balls. Let the balls come back to "chilled" temperature before baking.
Preheat oven to 375 degrees convection. Scoop the dough into balls and place onto a parchment lined baking sheet (my favorite linked here, affiliate link), about 12 cookies per baking sheet. Sprinkle the top with flaky sea salt if desired.
Bake cookies at 375 degrees convection for 6 minutes until cookies are puffed up and the edges are a little crispy. Let the cookies sit for about 5 minutes on the baking sheet to set up before removing. If your oven doesn't have a convection setting, preheat oven for 20 minutes and bake cookies at 400 degrees for 6-8 minutes.
Repeat with the remaining cookie dough and enjoy!