Position a rack in the lower third of the oven and preheat it to 325-degrees F. Spray a 9x5-inch loaf pan with nonstick cooking spray and line it with parchment paper with a couple of inches overhang on 2 opposite sides.
In a large bowl, stir together the mashed bananas, brown sugar, oil, buttermilk, eggs, rum (if using) and vanilla.
In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Pour the dry ingredients into the wet and fold until just blended. Pour the batter into the prepared pan.
Place the granulated sugar in a small bowl. Dampen your fingertips with water and work them into the sugar until it just begins to look like snow-if you pinch some, it should just barely hold together. Sprinkle the dampened sugar over the batter, aiming to get it clumped up together in spots.
Bake until a toothpick inserted into the center of the cake comes out clean, 60 to 70 minutes. Let cool for twenty minutes in the pan, then use the parchment paper to help lift the loaf out of the pan and cool completely on a wire rack. Store any leftovers in an airtight container for up to 5 days.