In a mixing bowl, use an electric mixer (or stand mixer with paddle attachment) to beat softened butter, brown sugar, and white sugar until creamy and white, about 3 minutes on medium/high speed.
Add vanilla and egg to the mixing bowl and mix well, for another 1-2 minutes.
Add in the sourdough discard, salt, and baking soda and mix until just combined.Sourdough discard consistency should be on the thinner side like pancake batter, not very thick. Thicker discard makes stiffer cookie dough that won't spread. Add a Tablespoon or two of water if needed!
Add the flour (start with 2.5 cups and add more if needed) and mix on low speed until just combined.
Add in oats and chocolate chips last, mixing on low/medium speed until they are fully mixed in.
Scoop the dough into balls onto a prepared baking sheet (12 or 13 per pan) about 1.5 inches apart. I use a 1.5 Tablespoon cookie scoop.
Bake for 10-11 minutes until puffy and flattened. Cool for at least 10 minutes before serving.
Notes
If your cookies don't spread: make sure you are measuring ingredients by weight! Some sourdough discards made with different flours can make the dough more stiff. Add a Tablespoon or two of water to the dough if needed. Store at room temperature for up to 3 days. Makes about 40 cookies. TO FREEZE: 1. Make recipe for dough as directed. 2. Scoop onto cookie sheet lined with parchment paper close together but so edges are not touching. 3. Freeze 4 hours or overnight, then place into a freezer safe bag to use later. To bake thawed: Place cookie dough balls in refrigerator overnight and then onto prepared cookie sheet. Preheat the oven to 350 F and bake as directed in recipe. To bake while frozen: Preheat oven to 350 F. Place frozen cookie dough balls onto cookie sheet lined with parchment paper or baking mat. Bake for 12-14 minutes or until edges are barely golden brown and the tops are beginning to crack. After 12 minutes check every minute until done.