Add your softened butter and brown sugar to your mixing bowl and mix on medium speed until soft and fluffy.
Add your sourdough starter, egg, and vanilla paste and continue to mix.
Meanwhile, mix your flour, oatmeal, baking soda, and salt in a separate bowl.
Change your mixer speed to low. Then very slowly add in your flour mixture to the bowl. Turn off the mixer when that is well incorporated.
Fold in your chocolate chips and raisins to the dough.
Cover the bowl with plastic wrap and place it into the fridge for 1 hour to 20 hours.
Preheat your oven to 375 degrees Fahrenheit.
Allow your bowl of dough to sit out for 15 to 30 minutes.
Then scoop out your dough into 3-4 inch scoops onto a baking sheet covered with parchment paper.
Allow this to bake for 14 to 16 minutes - or until the bottoms of your cookies are light brown.
Then place the cookies onto a cooling rack. Enjoy once cooled!
