Prep: Preheat oven to 180°C (350°F). Line a baking sheet with parchment.
Cream: Beat butter, brown sugar, and white sugar until creamy.
Mix wet: Add egg and vanilla; mix smooth.
Add starter: Stir in sourdough starter until combined.
Add dry: In a separate bowl, mix flour, baking soda, and salt. Add to the wet ingredients and fold gently.
Add chips: Fold in chocolate chips.
Chill: Refrigerate dough for 30 minutes (or longer for better flavor).
Shape: Scoop dough into balls and place on the baking sheet.
Bake: Bake 12–15 min, until edges are golden and centers still soft.
Cool: Rest on the pan a few minutes, then cool on a rack.