Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or lightly grease it.
In a large bowl, whisk together the all-purpose flour, rolled oats, brewer’s yeast, ground flaxseed, baking soda, baking powder, salt, and ground cinnamon until well combined.
In a separate medium bowl, mash the ripe bananas with a fork until smooth.
Add the brown sugar, vegetable oil, eggs, vanilla extract, and milk to the mashed bananas. Whisk until the mixture is smooth and well combined.
Pour the wet ingredients into the dry ingredients. Gently fold the mixture with a spatula until just combined. Do not overmix; the batter should be slightly lumpy.
Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven and let them cool in the tin for 5 minutes.
Transfer the muffins to a wire rack to cool completely before serving.