Melt the butter in a microwave or the stove, then let it cool. In a separate bowl mix the flour, salt and baking soda and set it aside. Add the sugars, vanilla, egg and sourdough starter to the melted butter. Mix it together well. The sourdough starter may be a bit tricky to mix in at first, but keep stirring until combined.Tip in the flour and stir it all together until combined.Stir in the chocolate chips. They may not stick very well to the buttery dough but they will once the sourdough cookie dough has chilled.
Chill the dough for at least 2 hours.
Preheat the oven to 170°C (338°F) fan-bake, or 190°C (374°F) regular oven. Line two cookie sheets with baking paper. Roll the cookie dough into 16 golf-ball sized balls. Keep the balls nice and high, don't flatten them.
Space the cookies about 3cm apart, and bake for approximately 10-12 minutes until the edges of the cookies are golden brown but the tops are still pale. Let them cool for 5 minutes on the baking sheet before placing them on a wire rack to cool further.
Notes
In the original recipe the all-purpose flour amount was 300g and the discard starter amount was 150g.