Preheat oven to 400 degrees. Whisk flour, baking powder, sugar, cocoa, and salt together. Mix eggs, butter, sour cream, and vanilla into mashed bananas. Gradually add milk to mixture.
Add wet mixture to dry ingredients, and mix gently until almost combined. Fold in chocolate chips. Do not overmix. Spray muffin tins with nonstick spray, and fill about two-thirds full. Bake 20-25 minutes or until test utensil comes out clean. Let cool slightly, remove from baking pan, and dust with confectioners' sugar before serving.