Preheat oven to 375°F and prepare a muffin tin by adding liners.
Mix together the flaxseed and water. Set aside for 10 minutes.
In a large bowl, cream together the butter, granulated sugar, and brown sugar. I use a hand mixer but a stand mixer also works.
Mix in the mashed bananas, vanilla extract, and flax egg.
In a separate bowl, mix together the flour and baking soda (and cinnamon if applicable.)
Fold the flour mixture into the wet ingredients until incorporated. Fold in the nuts.
Scoop into the muffin liners (I add enough of the batter to fill each liner about 3/4 of the way.) Top with more nuts if desired.
Bake for 22-25 minutes, or until a toothpick comes out clean.
After removing from the oven, let the muffins cool for about 15 minutes. Then finish cooling on a wire rack completely before removing the muffin liners.
Notes
You can use chocolate chips instead of walnuts, or go half and half!