Preheat oven to 350°F.
Line a muffin tin with liners.
In a bowl whisk together the dry ingredients: flour, protein powder, baking powder, baking soda, salt, cinnamon & walnuts.
In a separate bowl whisk together the wet ingredients: mashed banana, eggs, greek yogurt, melted oil, brown sugar, maple syrup & vanilla.
Add the wet ingredients into the dry. Gently fold until just combined (If batter feels too thick, add 1–2 tbsp milk. It should be thick but scoopable. Do not overmix!)
Fill muffin cups about ¾ full.
Sprinkle extra walnuts on top
Bake 18–22 minutes, until a toothpick comes out with moist crumbs.
Let cool 10 minutes before removing from pan.