Preheat oven to 350F. Grease or line a 12-cup muffin tin. Set aside.
In a large bowl, mash bananas until only small chunks remain.
Whisk in oil and brown sugar. Mix until well combined.
Whisk in eggs and vanilla until combined.
Whisk in the buttermilk until incorporated.
Add flour, baking soda, baking powder, salt, and cinnamon. Whisk until just combined. Do not over-mix.
Stir in nuts if using.
Divide batter between prepared muffin cups.
Bake for approximately 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean, and tops are turning golden brown.
Remove from oven and let cool in pan for five minutes.
Remove muffins from pan and serve warm or place on wire cooling rack to cool to room temperature.
Using overripe bananas (bananas that are at least half speckled) will give you sweeter muffins with stronger banana flavor.
Oils such as canola oil, melted coconut oil, and even light olive oil work well. However, if you use melted coconut oil, it's best to have your ingredients at room temperature to avoid clumps of coconut oil.
Full-fat or low-fat ok.
I recommend sifting or aerating the flour, spooning, and then leveling. Too much flour will give you dry or dense muffins.