Place softened butter and sugar into large mixing bowl. Mix using hand mixer or stand mixer on medium speed 1-2 minutes, or until butter mixture is pale yellow, light and fluffy.
Add in egg to creamed butter and sugar, continue to mix slowly
Add sourdough starter and vanilla extra
Mix in all-purpose flour, salt, and baking soda. Use a rubber spatula to ensure that everything is evenly mixed.
Incorporate the chocolate chips. Cover the bowl in plastic wrap and place it in the fridge for 30 minutes to an hour
Remove cookie dough from fridge and allow it come to room temperature. Preheat oven to 350°F/176°C while you are waiting. Cover baking sheet with parchment paper.
Roll 3 tablespoons of dough per cookie between your hands to create a ball. Slightly flatten on the pan.
Bake for 15 minutes or until the edges are a golden brown. Remove from oven and let rest for at least 5 minutes on pan before moving to a cooling rack.