Preheat oven to 350° Fahrenheit. Line a standard muffin tin with paper baking cups.
In a large mixing bowl, prepare the flax egg by mixing flaxseed meal and water together. Wait 5 minutes.
Peel the bananas. Add them in the bowl and mash them into a smooth paste.
Add the flour, sweetener, cinnamon powder, vanilla extract and baking powder.
Using a spatula, fold the wet and dry ingredients together until the dry bits of the flour have disappeared.
Stir in chopped Walnuts. Gently mix again.
Using a small scoop, portion the batter evenly into a greased muffin tin. Do not fill more than ½ of the tin.
Tap the pan twice gently on your work surface to get rid of any air bubbles.
Top the muffins with your choice of toppings if using.
Bake for 20 minutes, until the muffins have risen and set. A toothpick inserted into the center of a muffin should come out clean. The edges should be dark golden brown.
Remove from oven and let muffins rest in pan on wire rack for 5 minutes.
Remove from pan and let cool completely on rack. Devour!