Cube the butter. Brown in a skillet over medium heat, stirring frequently. Stop when you see the brown flakes collecting at the bottom and the butter begins to smell 'nutty'. (THIS IS BROWN BUTTER! Each one of those little flakes is an umami flavor explosion!) Let cool before adding to mixture.
Preheat oven to 350 F
In a bowl, whisk together the dry ingredients (flours, baking soda, corn starch, salt). Set aside.
In a stand mixer, beat together on a medium speed the cooled brown butter and sugars until light in color and fluffy (about 7 minutes).
Mix in the egg until combined.
Add in sourdough starter and vanilla. Mix until combined.
Add the dry ingredients in 1/2 a cup at a time, mixing until just combined. DO NOT OVERMIX!
Mix in the chocolate chips by hand until well dispersed.
Cover the bowl and place in the fridge for at least an hour, preferably overnight to get the most benefits of the fermentation of the dough.
Line baking sheets with parchment paper.
Using a cookie scoop or a tablespoon, scoop dough and roll into balls. Spread them out well on the cookie sheet so they have room to spread.
Bake at 350 F for 13-15 minutes, until you see the edges begin to turn golden. Do NOT overbake!
These stay fresh in a sealed container for about a week! They are also easily frozen in both cooked form or as uncooked dough balls. If you would like to cook them from frozen dough, pull them up to defrost for 30 minutes to an hour before baking.