1 1/2 cups semisweet or dark chocolate chips (I use Enjoy Life)
Place the butter and sugars in the bowl of a standing mixture and, using the beater attachment, mix on medium for 2 minutes. Scrape the sides of the bowl. Add the sourdough starter, vanilla and egg and mix on low until combined.
In a separate bowl, combine the flour, oats, baking powder, baking soda and salt. With mixer on low, slowly add the dry ingredients to the wet. Then add chocolate chips and mix until just incorporated.
Cover the bowl tightly and place in the refrigerator for 72 hours.
To bake: Preheat the oven to 350ºF and adjust the rack to the middle position. Using a 2-inch cookie scoop, scoop the dough into balls and put on a baking sheet lined with parchment paper. Gently press on each cookie to flatten it a bit (so that it doesn’t have a dome on top). Bake for 14-16 minutes until just golden brown on the edges.