Start by preheating your oven to 350°F (175°C). Grease and line a 9×9-inch baking pan with parchment paper.
In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until smooth.
Add the eggs, vanilla extract, and mashed bananas. Stir until everything is well combined.
In a separate bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon.
Gradually fold the dry ingredients into the wet ingredients. Stir until just combined. If you like, fold in the chopped walnuts.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 25–30 minutes or until a toothpick inserted into the center comes out clean.
While the brownies cool, whisk together the powdered sugar, milk, and vanilla extract to make the glaze.
Drizzle the glaze over the cooled brownies and cut them into squares.