Start by mixing your dry ingredients (flour, salt, baking powder and baking soda) in a bowl with a whisk or fork. Set aside.
In a separate bowl, whisk your egg, vanilla extract and sourdough starter until smooth. Set aside.
In your stand mixer fixed with the paddle attachment, add your cold, cubed butter, brown sugar and granulated sugar, and beat on low speed, until small crumbles appear. Add the chocolate chips and mix on low speed until combined.
Add your dry ingredients to the mixer, and mix on low-speed for 30 seconds. Then add your sourdough starter and egg mixture, and mix for another 30 seconds, until everything is just combined. Remove from stand mixer, and make sure everything is well incorporated by mixing with a spatula.
Now it is chill time! Place your dough in the fridge for a minimum of 2 hours, or up to 24 hours.
Once you are ready to bake, preheat your oven to 375°F (190°C) and line your baking sheet with parchment paper. Use a cookie scoop to ball your dough, and slightly smash them down once on your baking sheet. If desired, sprinkle the tops with sea salt (you won't regret this decision!)
Bake for 13-16 minutes, until the edges slightly start to brown, but the middle still looks a bit doughy. Allow the cookies to rest on the hot baking sheet for 5 minutes before transferring to a cooling rack.