Cream butter, cane sugar, and brown sugar in a mixing bowl until smooth.
Add the egg and mix well.
Mix in sourdough starter and vanilla extract until smooth.
Sift in flour, baking soda, and salt. Mix until just combined.
Fold in chocolate chunks.
Scoop the dough onto a baking sheet (about 18 cookies total).
Cover and refrigerate for at least 2 hours, preferably overnight.
Preheat oven to 175°C (350°F). Bake cookies for 12–15 minutes.
Let cool before serving. Store in an airtight container for up to 3 days or freeze unbaked dough for later.