Use a whisk or a fork to mix the bread flour, all-purpose flour, baking soda, baking powder and salt in a small bowl. Set aside.

In a separate small bowl, use a whisk or a fork to beat the egg, vanilla extract and sourdough starter until smooth. Set aside.

Place the cold, cubed butter, light brown sugar and granulated sugar into the bowl of a stand mixer with the paddle attachment. Beat on low-speed until the mixture forms small crumbles, 45-60 seconds. Add the chocolate chips and mix on low-speed 30-45 seconds. Scrape down the sides of the bowl.

Add the dry ingredients and mix on low-speed until the batter starts to become a crumbly mixture, 20-30 seconds. Pour in the egg mixture and mix on low-speed until the dough comes together and starts to pull away from the sides of the bowl, 20-30 seconds. Use a spatula to stir the batter, ensuring all of the ingredients have been fully incorporated.

Use a cookie scoop to portion the dough into 12 balls, (2 scoops per cookie) and place them on a parchment-lined baking tray. Cover the dough with a sheet of plastic wrap so they do not dry out. Chill the dough in the fridge for a minimum of 2 hours. (24 hours is best!)

Bake in a preheated, 375°F (190°C) oven for 15-17 minutes, or until the edges are golden brown and the center still appears slightly under-baked. Remove and allow the cookies to rest on the baking sheet for 5 minutes before transferring them to a cooling rack.