Preheat the oven to 350 degrees F. Spray a 24-cup mini-muffin tin with cooking spray.
Whisk together the flour, cocoa, baking powder, baking soda and salt in a medium bowl.
Whisk together the coconut oil and granulated sugar in a large bowl until well combined. Add the egg and vanilla extract and whisk until well mixed. Whisk in the milk, then add the flour mixture and whisk until just incorporated. Stir in the chocolate chips and walnuts.
Divide the batter evenly among the muffin cups. Bake until a wooden toothpick inserted into the center of the scuffins comes out clean, about 15 minutes. Cool in the tin on a cooling rack for 5 minutes and then remove from the tin and let cool completely. Dust with confectioners' sugar before serving.