Preheat oven to 375 degrees F (190 degrees C). Lightly spray a 9x5 inch loaf pan or 16 muffin cups with cooking spray. Stir the skim milk and lemon juice together in a glass measuring cup; let stand until curdled, about 30 minutes. Whisk together the whole wheat flour, cocoa powder, baking powder, baking soda, cinnamon, nutmeg, and salt; set aside.
Beat the eggs and sugar together with an electric mixer until smooth. Mix in the mashed bananas, the milk mixture, canola oil, and vanilla extract. Stir in the flour mixture just until all ingredients are moistened. Pour the batter into the prepared pan or muffin cups.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 1 hour for a loaf, or 30 minutes for muffins. Cool in the pan for 10 minutes before removing to cool completely on a wire rack.
