Preheat oven to 350. Spray a 9×5″ loaf pan with nonstick cooking spray and set aside.
Slice the bananas. Over medium-high heat, melt the butter. Add the bananas and brown sugar and saute for 4 minutes, stirring frequently. Remove from heat and cool for 5 minutes.
Place the banana mixture in a large bowl & beat with an electric mixer (or by hand with a whisk) on medium speed until smooth.
Mix the the buttermilk, oil, vanilla extract, and eggs in a small bowl. Set aside. Whisk together the flour, baking soda, and salt. Beginning and ending with the flour mixture, alternately add it to the banana mixture, mixing on low speed until just combined.
Pour half the batter into the prepared baking pan. Sprinkle on 1/3 cup brown sugar. Top with remaining batter, then sprinkle on the other 1/3 cup brown sugar.
Bake at 350 for 45-60 minutes or until golden brown on top and a pick inserted into the center comes out with moist crumbs clinging to it. Cool for 10 minutes in the loaf pan. Remove the bread from the pan and cool completely on a wire rack.
Meanwhile, make your dulce de leche. Pour the can of sweetened condensed milk into a glass pie dish & cover with foil. Place that pie dish in a larger dish (like a roaster) and pour enough hot water in to where it comes up halfway on the pie dish. Bake at 425 degrees for 1 hour.
Drizzle the warm dulce de leche over the cooled banana bread. Reserve any extra dulce le leche to spread on individual slices of bread.