Place rack in the middle of the oven and preheat to 350 degrees Fahrenheit. Grease a 9”x5” nonstick loaf pan with butter.
In the bowl of a standing mixer, set with the paddle attachment, beat the butter at medium speed until light and fluffy, about 3 to 4 minutes. Add the brown sugar and granulated sugar and continue beating until smooth. Add the eggs and vanilla and continue beating for a couple minutes.
In a bowl, combine the flour, baking soda, baking powder and salt. Reduce the mixer speed to low, and add the flour mixture ¼ cup at a time. Continue beating until well incorporated. You will have a very dry batter.
Remove a third of the batter and set aside in a separate bowl. Add the mashed bananas to the remaining batter in the mixer bowl and continue beating until the batter is fluffy and mixed. Pour the banana batter into the prepared pan.
Rinse the mixer bowl and paddle attachment. Add the reserved batter and the Dulce de Leche to the mixer bowl and beat at medium speed, for a minute, until blended. Reduce speed, pour in the boiling water and continue beating until completely smooth. Pour the dulce de leche caramel batter in the pan over the banana batter.
Place in the oven and bake for an hour, until the top is puffed up, golden brown and a toothpick comes out moist but not wet on a corner of the bread. Remove from the oven, let cool and flip out of the pan onto a platter. Garnish with confectioners’ sugar. Once it has cooled completely, cover it, so it retains and even gains more moisture.