Preheat oven to 350 degrees. Whisk together flour, cocoa powder, baking powder and salt in a bowl. Set aside. In a glass measuring cup, microwave ½ cup dark chocolate chips for 30-60 seconds, until melted. Allow to slightly cool. In a stand mixer bowl fitted with the paddle attachment, mix together brown sugar and eggs until fluffy. Slowly add in eggs, pumpkin, oil, milk and vanilla. Mix in melted dark chocolate. Whisk in dry ingredients until just combined. Fold in remaining whole chocolate chips. Pour batter into a loaf pan coated with cooking spray. Top with chopped walnuts. Bake for 35-45 minutes, until toothpick inserted in the center comes out clean. Allow to cool on a wire rack before slicing.