Whisk together the all-purpose flour, bread flour, salt, baking soda, baking powder, and instant espresso powder in a medium bowl. The espresso powder won't add coffee flavor—it actually deepens and intensifies the chocolate notes, which is a key flavor trick in these cookies. Set the dry mixture aside. Make sure your egg is at room temperature (about 70°F) before starting, as this helps it incorporate smoothly into the dough and creates a better texture.

In a separate bowl, whisk together the room-temperature egg, sourdough starter, and vanilla extract until well combined. The sourdough starter adds subtle tang and complexity that regular cookies don't have—it won't taste sour, but it rounds out the chocolate flavor beautifully. Set this mixture aside.

In a large bowl, beat the cold butter cubes with the light brown sugar and granulated sugar for 45-60 seconds using an electric mixer on medium speed. The butter doesn't need to be softened—beating cold butter with sugar creates a slightly grainier texture that leads to chewier cookies with better structure. Stop when the mixture is combined but not overly fluffy; we want controlled mixing here since we're using sourdough starter.

Add the chocolate chips to the butter-sugar mixture and mix for 30-45 seconds until they're evenly distributed. Pour in the dry ingredient mixture from Step 1 and mix for 20-30 seconds until just combined—don't overmix, as this keeps the cookies tender. Add the wet ingredient mixture from Step 2 and mix for another 20-30 seconds until the dough just comes together. The dough should look shaggy but cohesive.

Scoop the dough into 12 equal balls using a cookie scoop or spoon and place them on a parchment-lined baking tray. Cover the tray loosely with plastic wrap and refrigerate for at least 2 hours, but preferably 12-24 hours. This resting time allows the sourdough starter to develop its flavors, helps the dough firm up for better spreading control, and improves the overall texture of the baked cookies.

Preheat your oven to 375°F. Bake the chilled cookie dough balls for 15-17 minutes, until the edges are set and the centers look just barely underdone—they'll continue cooking on the hot tray after you remove them from the oven. Let the cookies rest on the baking tray for 5 minutes before transferring them to a cooling rack. I like to let them cool completely on the rack before eating, as this allows the crumb to set and the chocolate to firm up slightly, giving you the best texture.