Prep your pans: Line two baking sheets with parchment paper. Set the oven to 350°F (175°C). If your kitchen is warm, chill the raisins while you prep to keep them from getting overly soft.

Soak the raisins (optional but recommended): Cover raisins with hot water for 5–10 minutes, then drain and pat dry. This keeps them plump in the oven.

Cream the butter and sugars: In a large bowl, beat the butter, brown sugar, and granulated sugar until light and fluffy, about 2 minutes. Scrape the bowl.

Add egg, vanilla, and starter: Beat in the egg and vanilla until smooth. Mix in the sourdough starter until fully blended. The mixture may look slightly loose—that’s fine.

Whisk dry ingredients: In a separate bowl, whisk flour, cinnamon, baking soda, baking powder, and salt.

Combine: Add dry ingredients to the wet and mix on low just until no dry streaks remain. Stir in oats and raisins (and nuts or chocolate, if using). Do not overmix.

Chill: Cover and refrigerate the dough for 30–45 minutes. This helps control spread and concentrates flavor.

Scoop and shape: Scoop heaping tablespoons (about 1.5 tablespoons each) onto prepared sheets, spaced 2 inches apart. Gently flatten the tops for even baking.

Bake: Bake for 10–12 minutes until edges are set and lightly golden but centers look slightly underdone. They’ll firm up as they cool.

Cool: Let cookies rest on the sheet for 5 minutes, then transfer to a rack to cool completely. For chewier cookies, don’t overbake.