Preheat oven to 350°F.
Place the butter and sugar in the bowl of a standing mixer and, using the beater attachment, mix on medium for 2 minutes. Scrape the sides of the bowl as need.
Add the sourdough starter, vanilla, and egg. Mix on low until combined.
In a separate bowl, combine the flour, oats, baking powder, baking soda, and salt.
Slowly add the dry ingredients to the wet. Then add chocolate chips and mix until just incorporated.
Cover the bowl with plastic wrap and allow to ferment at room temperature for 6-8 hours, or in the refrigerator 72 hours.
When ready to bake, use a cookie scoop to scoop the dough into balls onto a greased baking sheet.
Use your fingers to gently flatten the cookies
Bake for 14-16 minutes until they are light golden brown.