Preheat an oven to 350°F. Grease a loaf pan with cooking spray.
In a bowl, stir together the flour, baking powder, salt and baking soda.
In a large bowl, mash the bananas with a fork. Add the sugar, butter, eggs, cinnamon and vanilla and mix with a wooden spoon until well blended.
Add the flour mixture to the banana mixture and stir well.
Pour half of the batter into the prepared pan, spreading it evenly with the wooden spoon. Spread the dulce de leche on top and pour the rest of the batter on top of the dulce de leche.
Put the pan in the oven and bake until a toothpick inserted into the center of the bread comes out clean, about 40 to 45 minutes.
Remove the pan from the oven, set it on a cooling rack and let cool for 10 to 15 minutes.
Turn the pan on its side and slip the loaf out onto the wire rack. Let the loaf cool for about 10 minutes before serving.