Preheat the oven to 175°C (350°F) and grease a 9×9-inch baking dish.

In a mixing bowl, combine mashed bananas, melted butter, granulated sugar, and brown sugar.

Add eggs and vanilla extract, mixing until smooth.

Stir in flour, baking soda, salt, and cinnamon until just combined. Fold in chopped walnuts if using.

Pour the batter into the prepared baking dish and spread evenly.

Bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

While the brownies are baking, prepare the glaze by melting butter in a saucepan over low heat.

Remove from heat and whisk in powdered sugar, milk, and vanilla until smooth.

Drizzle the glaze over the warm brownies and let them set before slicing.