Moist banana bars topped with a rich, nutty brown butter frosting—a cozy twist on classic banana bread perfect for dessert or a snack.

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Preheat oven to 350 °F (175 °C). Line a 9×9‑inch pan with parchment or grease well.
In a bowl, whisk flour, baking soda, salt, and cinnamon. Set aside.
In another large bowl, cream softened butter and both sugars until light and fluffy, about 2–3 minutes.
Add eggs one at a time, beating after each, then stir in vanilla extract.
Fold in mashed bananas and sour cream until combined.
Gradually add dry ingredients and stir until just incorporated—avoid overmixing.
Spread batter evenly in prepared pan. Bake 25–30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow bars to cool completely in the pan on a wire rack.
While bars cool, make brown butter frosting: in a small saucepan, melt butter over medium heat. Continue cooking, swirling pan until butter melts, foams and turns golden-brown with a nutty aroma (about 4–5 minutes). Remove from heat and let cool briefly.
Pour browned butter into a bowl, whisk in powdered sugar, vanilla, and enough milk/cream to reach spreading consistency.
Frost cooled banana bars, spread evenly; cut into squares. Serve!

Cool bars completely before frosting to prevent melting.
Use very ripe bananas for maximum sweetness and flavor.
Frosting can be made ahead and stored in fridge—bring to room temperature before spreading.
Add chopped walnuts or pecans to the batter or sprinkle on top for extra texture.
Store covered at room temperature for up to 3 days, or refrigerate up to 5 days.