Preheat your oven to 375°F. Line your baking sheets with a Silpat or parchment paper.
In a medium bowl, combine the flour, cinnamon, baking soda, and salt.
In a large bowl or your mixer bowl, cream together the butter and the white and brown sugar. Once it’s light and fluffy, add the egg, vanilla, and sourdough starter and mix thoroughly.
Add the dry ingredients into the wet and mix until the flour has just absorbed, do not overmix. If you’re concerned you may overmix, you can incorporate the dry ingredients using a spatula.
Fold in the oats using a spatula. If you are adding raisins or chocolate chips, fold them in now.
Allow the mixture to chill in the fridge for about an hour. You can also let it sit in the fridge for up to 24 hours before baking.
Scoop out 2 tablespoon balls of the cookie dough onto your cookie sheet, leaving about 2 inches between the cookies.
Bake for about 14 minutes or until the cookies are golden brown and slightly risen. Don't bake for longer, or the cookies will overcook.
Allow the cookies to cool slightly before transferring them onto a wire rack to cool fully.
