In a large bowl or bowl of an electric mixer, beat the butter, white sugar, and brown sugar together on medium-high speed until they are creamed together and light and fluffy. Add the eggs and vanilla extract, then scrape down the sides of the bowl and beat again on medium-high until evenly mixed.
1 cup butter, 1 cup granulated white sugar, 1 cup packed brown sugar, 2 large eggs, 2 teaspoons vanilla extract
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and espresso powder. Add these dry ingredients to the butter-sugar mixture and beat on low speed just until combined.
Add the chocolate chips and walnuts, and stir in by hand.
1 cup semi-sweet chocolate chips, 1 cup walnut pieces
Chill the dough for about 2 hours. If you leave the dough in the refrigerator for 3 hours or more, plan to let it rest at room temperature for 15-20 minutes to make it easier to scoop and roll. (See note about an option to scoop, then chill the cookies.)
When ready to bake, preheat the oven to 350 degrees F and line large baking sheets with parchment paper or silicone mats.
Scoop the dough into portions of roughly 1.5 Tablespoons each, then roll into smooth balls and arrange 2-3 inches apart on cookie dough sheets. Bake for 11-12 minutes, until the edges are set but the centers are still soft.
Press extra chocolate chips and walnuts into the cookie tops immediately after removing from the oven, if desired for looks. Let cookies cool for 5-10 minutes on the baking sheet, then transfer to a wire rack to cool completely. Enjoy!
Notes
Store cookies tightly covered at room temperature for 3-4 days.
If you prefer, you can scoop and roll the cookie dough into balls prior to chilling. Place the dough balls on a plate or baking sheet, chill for 2 hours, then bake as directed. Dough balls can also be frozen for up to 3 months and baked from frozen, adding 2-3 minutes to the baking time.