Heat oven to 350°F. Line an 8-inch square pan with parchment and lightly grease.
Whisk melted butter, brown sugar, and granulated sugar until glossy and smooth.
Add beaten egg and vanilla. Whisk until fully combined.
Stir in mashed bananas until the mixture looks cohesive.
In a bowl, whisk flour, baking powder, baking soda, salt, and cinnamon.
Fold dry ingredients into the banana mixture just until no dry spots remain.
Gently fold in walnuts or chocolate chips, if using.
Spread batter in the pan and smooth the top.
Bake 22–27 minutes, until edges are set and a toothpick shows moist crumbs.
Cool completely in the pan. Lift out, slice into squares, and serve.