In a medium saucepan, melt unsalted butter over medium heat, stirring constantly until it turns a deep golden brown, about 5-7 minutes. Let cool slightly.

In a mixing bowl, combine chopped walnuts, granulated sugar, and flour. Add cooled brown butter and mix until crumbly. Set aside.

In a large mixing bowl, mash ripe bananas until smooth. Add cooled brown butter and eggs, whisking until combined.

In another bowl, whisk together flour, baking soda, salt, and cinnamon. Mix well to ensure no lumps.

Fold the dry mixture into the wet ingredients until just combined without overmixing.

Preheat oven to 350°F (175°C) and line a loaf pan with parchment paper. Spoon batter into pan and smooth the top.

Distribute walnut streusel evenly over the top, pressing gently to adhere.

Bake for 50-60 minutes until a toothpick comes out clean. The top should be golden brown.

Once baked, cool in the pan for 10 minutes then transfer to a wire rack to cool completely before slicing.