Preheat the oven to 350ºF. Grease and line a 9x5 loaf pan with parchment paper, making sure the parchment is hanging over the sides of the loaf pan. This will help you easily remove the loaf after baking.
In a large bowl, cream butter and sugar together for about 2 minutes until smooth. Mash the bananas with a fork then stir them into the batter. Whisk in the egg and add in the vanilla. Mix, scraping down the sides of the bowl, until well combined.
Add the flour, baking soda, baking powder, and salt. Make sure the flour is not clumpy. Mix until just combined. Careful not to over mix the wet with the dry ingredients. Transfer the batter into the loaf pan.
Spoon 3-4 large dollops of the dulce de leche on top of the batter then using a knife gently swirl the dulce de leche into the batter to give it a marbled effect, about 2-3 times.
Bake for 55-65 minutes until a toothpick inserted in the center of the loaf comes out clean. Let the banana bread cool for at least 30 minutes before serving. Drizzle more dulce de leche on top of the bread. Slice and enjoy!
The nutritional info is an estimate. This recipe makes 1 loaf, about 12 slices, depending on how thick/thin you cut the slices.