To brown the butter: Cook butter in a saucepan on lowest heat setting, stirring occasionally. The butter will begin to bubble and foam. When brown milk solids are just visible, remove from heat. Transfer to a heatproof bowl. To roast the bananas: Preheat oven to 350° F. Place 4 whole, ripe bananas on a baking sheet. Bake for 10-15 minutes or until peels are blackened. Let cool completely, then cut the banana peels, remove the banana and mash. Set aside. Preheat oven to 350° F. Heavily spray 9 x 5-inch loaf pan; set aside. In a bowl, whisk together flour, baking soda and salt; set aside. With a hand mixer or stand mixer, cream butter and sugar for 2-3 minutes. Add eggs, beat to incorporate. Add flour mixture to the butter mixture. Mix until just combined. Add bananas, sour cream and vanilla. Mix to combine. Stir in nuts. Pour into prepared pan. Bake until tester comes out clean; approximately 50-60 minutes. Let rest 10 minutes before turning out on a rack to cool.