Preheat oven to 350 degrees. Prepare two baking sheets by lining with parchment paper or silicone mats. Set aside. Melt the chocolate in the microwave in 30-second increments until glossy, smooth, and melted. Set aside to cool. Whisk together the flour, cocoa powder, baking powder, and salt together until combined. Set aside.
In a bowl of a stand mixer, beat butter and sugars together until fluffy and light, about 3 minutes. Add eggs, one at a time and beat together. Add vanilla and cooled chocolate and beat just until combined. Add in all the dry ingredients and gently pulse the dough until it starts to come together. Beat for 5 more seconds. Add in walnuts and chocolate chunks. beat for 5 seconds until mixed throughout.
Take 1 tablespoon of dough and roll in a dough with your palms. Place on baking sheet. Repeat with the rest of dough, spacing the cookies about 1.5 inches apart. They will not spread much while baking. You can flatten them out a little if you want the baked cookies to be less round.
Bake in preheated oven for 13-15 minutes, or until the edges begin to crack and reach the centers. Remove from the oven and cool for 10 minutes before serving.