Preheat oven to 350°F (175°C) and grease a 12-cup muffin tin or line with paper liners.
In a large bowl, whisk together the mashed bananas, peanut butter, maple syrup, egg, and vanilla extract until well combined.
In a separate medium bowl, whisk together the whole wheat flour, baking soda, salt, and cinnamon.
Gradually fold the dry ingredients into the wet ingredients until just combined.
Stir in the almond milk until the batter is smooth, being careful not to overmix.
Divide the batter evenly among the 12 muffin cups.
Bake for 18 to 22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.