Preheat your oven to 375°F (190°C) and line a 12-cup muffin tin with liners or spray with cooking spray.

Peel and mash the bananas in a small bowl with a fork until mostly smooth with a few small chunks remaining.

In a large mixing bowl, whisk together the vegetable oil, granulated sugar, and egg for about 1 minute, until pale and slightly frothy.

Stir in the mashed bananas, vanilla extract, and buttermilk until well combined.

In a medium bowl, whisk together the all-purpose flour, baking soda, salt, and ground cinnamon.

Gently fold the dry ingredients into the wet mixture with a spatula until just combined and no streaks of flour remain.

Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.

Bake for 20 to 22 minutes, until the tops are golden brown and a toothpick inserted into the center comes out clean or with a few moist crumbs.

Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before serving.