Preheat the oven to 350 degrees F. Line a muffin pan with paper liners and set aside.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon. Set aside.
Peel the bananas and mash with a fork. Place the mashed bananas in a large bowl. Add the brown sugar, coconut oil, egg, Greek yogurt, and vanilla extract. Stir until well combined. Slowly stir in the dry ingredients. Mix until just combined.
Fill muffin liners three-quarters full. Sprinkle with turbinado sugar, if desired. Bake until toothpick inserted into center comes out clean, about 23 minutes. Transfer to wire rack to cool. Store, covered, at room temperature.