In a small saucepan, melt the butter over medium heat. Continue cooking, swirling the pan, until the butter turns golden brown and smells nutty (about 5–6 minutes). Remove from heat and let cool slightly.
Preheat your oven to 175°C (350°F). Grease and line a 9×5-inch loaf pan with parchment paper.
In a large bowl, whisk together the mashed bananas, brown sugar, granulated sugar, eggs, bourbon, and vanilla. Pour in the cooled brown butter and whisk until smooth.
In another bowl, whisk together flour, baking soda, cinnamon, and salt. Gently fold the dry ingredients into the wet mixture until just combined — do not overmix.
Fold in chopped pecans or walnuts for texture and flavor.
Pour the batter into the prepared loaf pan. Bake for 55–60 minutes, or until a toothpick inserted into the center comes out clean.
Let the banana bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing.
