Prep: Preheat oven to 375 F. Line 2 baking sheets with parchment paper. Set aside.
Brown butter: Melt the butter in a small saucepan over medium heat. Heat the butter until it begins to smell nutty and turns the color of graham crackers, about 5 minutes. Transfer the butter to a large mixing bowl and allow it to cool.
Stir in the sugars: Whisk in the cinnamon, brown sugar and granulated sugar until completely incorporated. It may look a little grainy, this is totally ok.
Make the dough: Add in the sourdough discard, egg, oil, all-purpose flour, baking soda and salt. Stir the mixture using a spatula until no traces of discard and egg or dry bits of flour remain.
Fold in oatmeal and mix-ins: Fold in the oatmeal, raisins and chocolate chips until everything is well distributed.
Portion out the dough: Using a large cookie scoop, portion out the dough (3 tablespoon portions) and place them on the prepared baking sheet, about 1-inch apart to allow them to spread. Flatten the cookies lightly.
Bake: Bake the cookies for 10 to 12 minutes or until the tops of the cookies look dry and the edges look set. Allow the cookies to cool completely on the baking sheet. Don’t move your cookies too soon or they may break apart.
Enjoy and store: Enjoy them warm, once the cookies have set completely. These cookies get even better once they have cooled completely and their flavor has fully developed. Store leftover cookies in an airtight container for up to 1 week.