Prepare your muffin pan with either paper liners or spray with non-stick spray. Preheat the oven to 350 degrees.
You're going to want ripe bananas as they will taste the best in really any banana recipe. Mash the bananas using a fork and set aside.
In a small bowl, In a mixing bowl, whisk together the Stevia, flour, baking soda, and baking powder. Set aside.
In a bowl of a stand mixer, add the mashed bananas, beaten egg, and the cooled coconut oil; mix until combined and then add in the vanilla extract. Give it all a stir to combine again.
Add the dry ingredients gradually and mix after each addition. Don't over mix.
Using a muffin scoop, fill the muffin cups about 2/3 of the way full. You will have enough batter for exactly 12 muffins. Bake for 20 - 25 minutes, or until a toothpick inserted in the center of the muffin comes out clean. Allow to cool before serving.