In a large bowl, whisk together the cake flour, bread flour, baking soda, salt, and espresso powder (if using). Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, beat the cold, cubed unsalted butter on medium speed for 1-2 minutes until slightly softened but still firm. Add the granulated sugar and dark brown sugar and beat on medium-high speed for 3-5 minutes, scraping down the sides as needed, until the mixture is light, fluffy, and noticeably paler.

Add the cold eggs one at a time, beating well after each addition until fully incorporated. Mix in the vanilla extract. The mixture may look slightly curdled, which is normal.

Reduce mixer speed to low. Gradually add the dry ingredients to the wet ingredients, mixing only until just combined and a few streaks of flour remain. Be careful not to overmix.

Remove the bowl from the mixer and, using a rubber spatula, fold in the dark chocolate chunks and toasted walnuts until evenly distributed. Ensure no dry pockets remain, but avoid overmixing the dough.

Divide the dough into 12 equal portions, each weighing approximately 6 oz (about 170g). Roll each portion into a rough ball, but don't over-compress. Place the dough balls on a parchment-lined baking sheet or plate. Cover loosely with plastic wrap and refrigerate for at least 2 hours, or preferably overnight (up to 24-48 hours). Chilling is crucial for flavor development and cookie structure.

Preheat your oven to 400°F (200°C). Line two large baking sheets with parchment paper. Arrange 3-4 chilled cookie dough balls on each baking sheet, leaving ample space between them. For the signature Levain look, slightly pull apart the dough balls in a few places to create craggy, uneven edges before baking.

Bake for 13-17 minutes, or until the edges are golden brown and set, but the centers still look soft, puffy, and slightly underbaked. The residual heat will continue to cook them as they cool.

Remove the baking sheets from the oven and let the cookies cool on the sheets for 10-15 minutes before carefully transferring them to a wire rack to cool completely. The cookies are best enjoyed warm, ideally within a few hours of baking.