Preheat oven to 425 degrees. Line a standard 12-count muffin pan with cupcake papers then set aside.
Add mashed bananas, brown sugar, oil, eggs, vanilla and milk to a large mixing bowl then whisk vigorously until combined.
In a separate bowl, sift together flour, cinnamon, baking soda, baking powder and salt.
Add dry ingredients to wet then stir with a wooden spoon until just moistened and flour is still visible. Do not overmix.
Remove 2 cups of batter and place in a separate bowl then whisk in cocoa powder until smooth and lump free. Note: batter will be thick.
Place one spoonful of chocolate batter into each muffin liner. Reserve the remaining chocolate batter for the muffin tops.
Distribute the banana batter evenly among 12 muffin liners then top each with an equal amount of the remaining chocolate batter.
Gently swirl the two batters together with a toothpick then bake for 5 minutes at 425 degrees. Keeping the oven door closed, reduce temperature to 350 degrees then bake for 15 additional minutes or until a toothpick inserted into the center of a muffin comes out clean. Do not overbake or muffins will be dry.
Remove muffins from the oven and cool in the muffin pan for 5 minutes. After 5 minutes, remove muffins from the pan and place on a wire rack to cool to room temperature.
Banana muffins can be stored in an airtight container at room temperature up to 3 days.