Preheat oven to 350°F and line a standard 12-cup muffin pan with paper liners.
Whisk all dry ingredients in a medium bowl and set aside.
Into another larger bowl, add all wet ingredients and whisk until very smooth. Add the dry ingredients and whisk until just combined (DO NOT OVER MIX).Tip: To mash bananas, peel them and place in a shallow bowl. Using a potato masher or the back of a large fork, press down on the bananas, until completely mashed and lump free.
Using a rubber spatula, stir in ½ cup chopped walnuts and 2 tablespoons water, until just mixed (DO NOT OVER MIX).
Divide batter evenly into the prepared muffin pan (remember, these are big muffins, so the batter will be almost full in the pan) , and sprinkle remaining chopped walnuts on top of the batter.
Bake for 28 to 32 minutes or until a tooth pick inserted into the middle of a muffin comes clean. Place pan on a wire rack to cool completely.Pro tip: Over mixing the batter will make the muffins to be tough heavy, dense or rubbery.
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Notes
Banana – Use over ripe bananas (dark yellow skin with lots of brown spots).To use greek yogurt - Use about ¾ cup plain greek yogurt and ¼ cup water instead of 1 cup regular yogurt.Vegetable oil – Use a flavorless oil like vegetable or corn oil.