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.