Preheat oven to 350°F. Grease and line an 8x8-inch baking pan with parchment paper.
In a mixing bowl, stir together the mashed bananas, melted butter, brown sugar, egg, and vanilla.
In another bowl, whisk the flour, baking powder, baking soda, salt, and cinnamon.
Add the dry ingredients to the wet ingredients and stir until just combined.
Pour the batter into the prepared pan and spread evenly.
Bake for 25–30 minutes or until a toothpick inserted in the center comes out clean.
Cool completely before cutting into squares.