- Preheat the oven to 350 degrees.  
- In a large bowl, beat together your brown sugar, white sugar, butter, and mashed bananas until well combined. 
- Add in the eggs, and stir until just smooth.  
- Whisk together the flour, cinnamon, baking powder, baking soda, and salt in a separate bowl.  
- Alternate adding half the dry ingredients, then half the buttermilk, then the rest of the flour mixture, and lastly the buttermilk. Mixing after each until just combined.  
- Pour batter into a 9x13 pan sprayed with nonstick spray. Spread evenly.  
- Bake for 33-38 minutes, or until a toothpick inserted in the middle comes out clean.  
- Remove from the oven and place cake pan on a wire rack to cool completely. Place in the fridge after 30 minutes to speed it up, if desired.