Prep pan and heat the oven: Place a rack in the middle of the oven and heat it to 350 degrees Fahrenheit. Butter and flour a standard 12-cup muffin tin, coat with nonstick cooking spray, or line with paper liners.
Toast the Walnuts: Place the walnuts on a baking sheet and toast for 8 to 10 minutes until lightly toasted. Set aside to cool before chopping.
Whisk Dry Ingredients: In a medium bowl stir together the flour, baking soda, baking powder, salt, cinnamon, and set aside.
Beat Eggs & Sugar: Using a stand mixer fitted with a whisk attachment, beat the eggs and sugar on medium speed for approximately 5 minutes, or until light and fluffy. With an electric hand mixer this will take approximately 8 to 10 minutes.
Add the Oil: With the mixer on low speed, slowly drizzle in the coconut oil. Do not pour it in all at once, you want to add it slowly so it can incorporate with the eggs and not deflate the air you beat into them.
Break Down Bananas: In a medium microwave-safe bowl, mash all but one of the bananas completely with a fork and microwave for approximately 60 seconds or until hot.
Add Remaining Banana, Yogurt & Vanilla to Puree: With a small whisk, add the yogurt and vanilla to the banana puree and which to combine. Mash the remaining banana and stir it into the banana mixture.
Add Banana Mixture to Egg Mixture: Mix until just combined on a low speed.
Add Dry Ingredients to Wet: Fold the dry ingredients, nuts, and chocolate into wet ingredients by hand with a rubber spatula until thoroughly combined. There should be no streaks of flour.
Pour Batter into Prepared Pan: Using a small ice cream scoop or spoon, fill each muffin tin to the brim (be careful not to over-fill).
Bake the Muffins: Bake for 25 to 35 minutes until the muffins are golden brown on top and spring back lightly to the touch when you press them in the center. Allow the muffins to cool on a wire rack for approximately 10 minutes, and then remove them from the pan to cool entirely.
Enjoy & Serve! The muffins are best enjoyed the same day, but they can be stored at room temperature in an airtight container for up to 2 days. Freezing: You can also store these in the freezer, tightly wrapped in plastic wrap for up to a month. To reheat from frozen, bake in an oven for 300 degrees Fahrenheit for 8 to 10 minutes.