Preheat the oven to 350°F. Line two large baking sheets with parchment paper. Set aside.
In a large mixing bowl, whisk together flour, cocoa powder, baking powder, and salt.
In a separate large mixing bowl or a stand mixer with a paddle attachment, cream the butter, granulated sugar, and brown sugar together until creamy for about 4-5 minutes. Beat in the eggs and vanilla until combined.
Add dry ingredients to the wet. Gently mix using a rubber spatula until combined. Fold in the chocolate chips and chopped walnuts.
Scoop out the dough using an ice-cream scoop and place on the prepared baking sheets, leaving about 2 inches between each cookie. Bake in the preheated oven for about 9-11 minutes or until the edges set.
Remove from the oven and let cool for 5-10 minutes on a baking sheet. Then, transfer to a cooling rack.