In the bowl of a stand mixer fitted with a paddle attachment, cream butter. Add brown sugar and granulated sugar. Mix until smooth. Scrape down bowl as needed. Add egg and vanilla extract. Mix until combined.
In another bowl, whisk together flour, salt, baking powder, baking soda, and espresso powder. With the mixer running on low, slowly add in dry flour mixture.
Before dough is throughly mixed add in rolled oats, walnuts, and dark chocolate. Fold to combine until toppings are evenly distributed.
Chill for dough for 30 minutes. Divide cookie using an ice cream scoop.
Preheat oven to 350 degrees F. Bake for 15-17 minutes, rotating baking sheet midway through baking. Allow to cool in pan for 3 minutes before transferring to wire rack to cool.