The first step is to brown the butter: Add the butter into a small saucepan and cook over medium heat whisking frequently and stirring until the butter turns a light amber color and small brown solid pieces begin to form on the bottom.
Remove the pot from the heat and transfer the brown butter into a small bowl or container. Cover it and refrigerate until it becomes 45 minutes.
Once the butter has chilled, remove it from the fridge and combine it with the sugars in a mixing bowl of a stand mixer.
Mix using the paddle attachment and beat the mixture until evenly combined for about 3 minutes. The brown butter mixture will end up having a wet, and sandy appearance.
Add the sourdough starter, egg, and vanilla, and mix until fully incorporated.
In a separate bowl, add the flour, salt, baking soda, and baking powder, and mix together using a whisk or a fork.
Add the flour mixture to the brown butter-sugar mixture, and mix just until the ingredients come together.
Fold in the chocolate chips until well combined in the cookie batter.
Scoop about 50 grams of cookie dough for each piece and place them onto a parchment paper-lined baking sheet, no need to separate them at this point.
Place the cookies into the refrigerator for two hours or freezer for 30 minutes to allow the batter to firm up.
Preheat the oven to 350°F (175°C), and line another baking sheet with parchment paper.
Remove the scooped cookies from the fridge, then place them onto the baking pans separating them about 2 1/2 inches apart. Press some chocolate chunks onto the top, about 3 to 4 pieces.
Bake the brown buttered sourdough cookies for 16-18 minutes, or until golden brown around the edges. The top of the cookies should be soft when they are fully baked.
Remove the cookies from the oven and let them cool on the pan for 5 minutes, then transfer them onto a cooling rack.
Enjoy!