Preheat oven to 400 degrees Fahrenheit. Line a muffin tin with parchment paper liners.
In a large bowl, combine the mashed bananas, sugar, and milk. Stir until completely combined.
Add the flour, baking powder, and ground cinnamon. It will look like too much flour but just keep mixing. Fold until just combined (so that no more flour is seen). The batter will seem thick and sticky.
At this point, you can add whatever customizations you want. Nuts, chocolate chips, dried fruit, etc. all work!
Fill each muffin cup all the way to the top (stacked very tall if you can). Bake for 8 minutes at 400 degrees Fahrenheit.
Then bring the oven down to 350 degrees and bake for another 15 minutes. The muffins should be golden brown and a toothpick inserted comes out clean.