Mash the ripe bananas in a mixing bowl until mostly smooth with only a few small lumps remaining.

Whisk in the egg, maple syrup, avocado oil, and vanilla extract until the mixture looks glossy and well combined.

In a separate bowl, whisk together the flour, protein powder, baking powder, baking soda, salt, cinnamon, and nutmeg.

Add the dry ingredients to the wet ingredients and gently fold until no large pockets of flour remain.

Fold in the chopped walnuts, distributing them evenly throughout the batter.

Divide the batter evenly among the muffin cups, filling each nearly to the top for nicely domed muffins.

Place the pan in the oven and bake for 5 minutes to encourage a tall rise.

Reduce the oven temperature to 175°C without opening the oven door and continue baking until the tops are golden and a toothpick comes out clean.

Remove the muffins from the oven when they feel springy to the touch and smell fragrant with banana and cinnamon.

Allow the muffins to cool in the pan briefly before transferring them to a cooling rack.

Let them cool completely before storing or serving.