Preheat the oven to 350 degrees F and butter and flour your bread loaf pan.
With a potato masher, mash the bananas until mostly smooth.
In the bowl of a stand mixer, cream together softened butter and 1 cup of sugar for about 1 minute until fluffy.
Add eggs one at a time, vanilla, mashed bananas and buttermilk. Mix until combined.
Sift together flour, baking soda and salt. With the mixer on low, slowly add the flour mixture and mix until just incorporated.
Pour the batter into the prepared bread loaf pan. Evenly sprinkle 1 tablespoon of sugar over the top and bake for 50-60 minutes until a toothpick inserted in the center comes out clean.
Remove from the oven and let it cool for 10 minutes. Loosen around the edge with a butter knife and carefully invert the buttermilk banana bread onto a plate.
Slice and serve warm with butter.