Preheat oven to 350°F (177ºC) and prepare parchment-lined 8½ x 4½-inch (1.5 litre) glass loaf pan or grease the pan generously.
In a large bowl, mash bananas completely then whisk in eggs, butter, buttermilk, vanilla, sugar and brown sugar. Add flour, baking soda, salt and cinnamon and continue mixing until well combined and the batter is smooth.
Pour the batter into prepared loaf pan, pressing it into the corners and smoothing out the top with a spatula or the back of a wooden spoon if needed. The batter will almost fill the pan and that’s okay!
Bake on the middle rack until a tester comes out clean, about 60-65 minutes depending on your oven, so begin checking every 5 minutes around the 60-minute mark. Remove pan from oven and let cool for 20 minutes. Using parchment edges, remove loaf from pan and transfer to a wire cooling rack. Let cool for 20 minutes more before slicing and enjoy!