Preheat the oven to 375°F. Spray insides of muffin pan with non-stick cooking spray.
Combine flour, baking powder, baking soda, cocoa, sugar, chocolate chips and walnuts in a large bowl. Combine milk, oil, egg and vanilla in a small bowl. Stir the dry and wet ingredients together until slightly lumpy.
Spoon batter into the prepared muffin pan. Sprinkle the remaining 1/4 cup nuts on top of muffins and bake for 20 minutes or until a toothpick inserted into the center of each muffin comes out clean. Let sit in pan for 5 minutes then turn muffins out onto a rack to cool.