• Preheat the oven to 350°F/175°C.
• Line a sheet pan with a sheet of parchment paper.
• Cream 113 g (½ cup/1 stick) butter and 200 g (1 cup) brown sugar in a large bowl with a hand mixer on low speed, gradually increasing to medium speed for 2 minutes until it's light and fluffy.
• Add the remaining wet ingredients to the butter mixture, 1 egg, 125 g (1/2 cup) sourdough starter, and 1 tsp vanilla extract. Mix until combined.
• Whisk 125 g (1 cup) all-purpose flour, 1/2 tsp salt, 1/2 tsp baking soda, 1 tbsp espresso powder, and 50 g (1/2 cup) cocoa powder in a separate bowl. Add 160 g (1/2 cup) chocolate chips to the bowl with the dry ingredients, tossing to coat the chips with flour. 
• Add the dry ingredients to the wet ingredients and mix until just combined.
• Scoop cookie dough with a 2 ounce cookie scoop and place on the prepared baking sheet 2 inches apart.
• Bake the cookies at 350°F/175°C for 11-12 minutes until the edges are set and the centers are still soft.
• Rest the cookies on the cookie sheets for 2 minutes before removing to a wire rack to cool.

Long Ferment Option

• Cover the bowl of cookie dough and refrigerate for up to 24 hours to give the bacteria and yeast in the sourdough a chance break down the phytic acid. The longer the cookie dough sits, the more tangy flavor develops in the cookie. Add a minute or two to your bake time when using cold cookie dough.