Preheat the oven to 350 degrees and put a rack on the middle shelf. Butter and flour a 9 by 5 by 3-inch loaf pan.
In a small bowl, mash the bananas with a fork, leaving a bit of texture to them.Set aside.
In a large bowl, whisk together the flour, baking soda, baking powder and salt. Set aside.
In another large bowl, use a hand electric mixer or stand mixer to cream the 1/2 cup of butter and sugar together until light and fluffy.
Add the eggs, 1 at a time. Stir in the mashed bananas, sour cream and vanilla and beat until just combined.
Add the dry ingredients and gently stir in pecans.
Pour the batter into the prepared pan.
Bake for 1 hour and 10 minutes. Let cool for 5 minutes in the pan then turn out onto a wire rack to finish cooling.