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

2. Cream the butter, brown sugar, and granulated sugar for 2–3 minutes until light and fluffy.

3. Add the egg, vanilla extract, peppermint extract, and sourdough starter; mix until smooth.

4. In a separate bowl, whisk together the flour, baking soda, and salt.

5. Gradually fold the dry ingredients into the wet mixture until just combined.

6. Toss peppermint chips in a tablespoon of flour, then fold them and the white chocolate chunks into the dough.

7. Cover and refrigerate the dough for 30–60 minutes to firm and develop flavor.

8. Portion dough into 1.5-inch balls and place 2 inches apart on the prepared sheets. Flatten slightly.

9. Bake for 10–12 minutes until edges are lightly golden and centers are soft.

10. Cool on the baking sheet for 5 minutes, then transfer cookies to a wire rack to cool completely.