In a pot or pan (I use a cast iron skillet) brown the butter.
While the butter is still hot, combine with the sugars in a large mixing bowl.
Let the butter/sugar mixture cool. This usually takes about an hour.
Once cooled, add the egg yolks and beat using a hand mixer.
Add the sourdough starter and vanilla. Mix to combine.
Mix in the dry ingredients and chocolate chips. Do not overmix.
*Depending on the hydration of your starter, you may need to add slightly more flour. The dough should be soft but not terribly sticky.*
Chill dough in fridge for at least an hour.
Preheat oven to 350°.
Once chilled, measure out your dough balls using a scale so they're all the same size. (This is optional, you can just scoop them).
Bake for 12 minutes or until the tops are just slightly browned.
Drop the cookie sheet aggressively on the counter or drop each cookie onto your cooling rack. This causes the middle to cave in and gives you a really pretty cookie.