Preheat the oven to 350 F.
Grease a 9 x 13 inch baking pan with butter or cooking spray. You can also line the bottom of the pan with parchment paper.
In a medium bowl, mash the bananas with a fork until smooth. Set the bananas aside.
In another medium bowl, whisk together the flour, baking soda, baking powder, cinnamon and salt. Set aside.
Using a stand mixer with the paddle attachment, cream together the butter, brown and white sugars on medium speed, until fluffy, about 2 to 3 minutes.
Beat in the eggs and vanilla until well combined, scraping down the sides as needed. Beat in the bananas.
Reduce speed to low. Starting with the dry ingredients, alternate adding the flour mixture and buttermilk in 3 instalments each, being careful to only beat until just incorporated. The batter will be lumpy.
Pour the batter into the prepared pan. Drizzle 1/2 cup of the caramel sauce over the batter and swirl around with a butter knife. Bake for 35-45 minutes, until a toothpick inserted in the centre comes out clean. Set the pan on a wire rack to cool completely.