Prepare flax egg. Whisk water and flax together and set aside to gel for approximately 10 minutes. Preheat the oven to 375 and either grease a muffin tin or place liners.
In a medium sized bowl, mash banana until no clumps appear.
Add in baking soda, and remaining wet ingredients. Whisk vigorously to combine.
Start adding in dry ingredients, stirring as you go. Once everything is mixed add chopped walnuts and chocolate chips. Use an ice cream scoop (or spoon) to fill up liners 3/4 of the way.
If you have a small cookie cutter, cut fun designs in the 12 slices of banana and place on top and slightly press into the batter. Take a handful of chocolate chips and randomly press 3 more into the top of the muffin.
*Macros are provided as an estimate for convenience. Your final numbers will vary depending on ingredient brands, substitutions, and how you portion the recipe.