Place some of the dry ingredients (flour, baking soda, corn starch, salt and chocolate chips) into a large mixing bowl and combine. Set this aside.
Now in a separate bowl, combine sourdough starter, melted butter, brown sugar, white sugar, egg + egg yolk and vanilla extract. Stir together until they resemble runny caramel.
Now pour the wet ingredients into the dry ingredients and stir to form the cookie dough.
Cover the bowl with plastic wrap and place into the fridge for 12 to 24 hours.
The next day, when you are ready to bake the cookies, preheat the oven to 165C (330F) and line two cookie sheet with parchment paper.
Use a cookie scoop or large spoon to scoop out balls of dough. I have used 30g dough balls for this recipe. Place the dough balls onto the prepared baking sheets.
Place the cookies into the oven for 12 to 13 minutes at 165C (330F) or until they are golden brown on top but still molten in the centre.
Remove from the oven and carefully transfer to a wire rack to cool. The cookies will continue to bake for a few minutes after they are removed from the oven and will firm up as they cool.