1. Preheat oven to 350°F (175°C) and line baking sheets with parchment paper.

2. Brown the butter in a light-colored pan over medium heat until amber with toasted flecks, pour into a bowl, and let cool 10 minutes.

3. In a mixer, beat cooled brown butter with brown sugar and granulated sugar until creamy, about 2–3 minutes.

4. Add eggs one at a time, then vanilla extract and peanut butter, mixing until smooth.

5. Fold in sourdough starter until just combined.

6. Whisk flour, oats, baking soda, and salt in a separate bowl.

7. Gradually add dry ingredients to wet mixture in batches, stirring until just combined.

8. Fold in chocolate chips and any optional mix-ins.

9. Optional: chill dough for 30–60 minutes for better flavor and less spread.

10. Scoop 2-inch dough balls onto prepared sheets, spacing 2 inches apart.

11. Bake 10–11 minutes for soft centers or 12–13 minutes for chewier edges, rotating sheets halfway.

12. Cool on baking sheet 5 minutes, then transfer cookies to a wire rack to finish cooling.