1. Cream the Butter and Sugars: In a large mixing bowl, using an electric mixer, beat the softened butter, granulated sugar, and brown sugar together until the mixture is light, fluffy, and pale in color. This usually takes about 2-3 minutes. Scrape down the sides of the bowl as needed to ensure everything is well combined.

2. Add Wet Ingredients: Beat in the egg and vanilla extract until fully incorporated. Then, add the sourdough starter discard and mix on low speed until just combined. The mixture might look a little curdled at this stage, but don’t worry, it will come together.

3. Combine Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking soda, and salt. Make sure there are no lumps in the flour mixture.

4. Gradually Add Dry to Wet: Slowly add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough once the flour is added, as this can lead to tough cookies. Stop mixing as soon as no dry streaks of flour remain.

5. Fold in the Mix-ins: Gently fold in the chocolate chips and rolled oats using a spatula or a wooden spoon until they are evenly distributed throughout the dough.

6. Prepare for Baking: Preheat your oven to 375 F (190 C). Line a baking sheet with parchment paper. Drop rounded tablespoons of dough onto the prepared baking sheet, leaving about 2 inches between each cookie. You can use a cookie scoop for uniform size.

7. Bake Until Golden: Bake for 10-12 minutes, or until the edges are lightly golden brown and the centers are still slightly soft. The cookies will continue to set as they cool.

8. Cool and Enjoy: Remove the baking sheet from the oven and let the cookies cool on the sheet for 5 minutes before transferring them to a wire rack to cool completely. This helps them firm up without breaking.