Heat the oven to 375°F. Line muffin pans with muffin baking cups.
Peel 5 of the bananas and place in a microwave safe bowl. Cover the bowl with plastic wrap and fold one edge back to vent and allow steam to release. Microwave on high until bananas release liquid, about 5 minutes. Note: If using defrosted, frozen bananas, it will take less time.
Transfer bananas to a mesh strainer set over a bowl. Allow the bananas to drain about 5 minutes, stirring if necessary. While the bananas drain, combine flour, baking soda, baking powder, and salt in a large mixing bowl. Slice fresh banana into 1/4-inch slices and chop nuts, leaving some of the nuts coarsely chopped for the topping.
Once the bananas have released their liquid, transfer the liquid to a small saucepan and then transfer bananas to the bowl the liquid was in. Simmer liquid in the pan for about 5 minutes to reduce to about 1/4 cup. Add the butter to the liquid to melt.
Stir the concentrated banana liquid with butter into the bananas and mash with a fork to combine. Whisk in the eggs, sugar, and vanilla until well blended.
Pour banana mixture into flour mixture and stir only until flour is moistened. Fold in walnuts and scoop batter into muffin cups, filling 3/4 full. Place sliced banana on top of muffins and sprinkle walnut pieces around banana, as desired.
Bake for 18 minutes, or until a toothpick inserted in the center comes out clean. Remove from oven and carefully transfer muffins to a wire rack once they are cool enough to handle. Serve warm and cool completely before storing or freezing.
TIPS for using fresh bananas:
Start with very ripe bananas, all those brown spots are a good thing and produce a stronger banana flavor.
When using fresh bananas, it may take a minute longer for the juices to release in STEP 1.
TIPS for using frozen bananas:
Allow the bananas to defrost completely.
Remove the peeling by making a long slice down the length of the banana with a sharp knife. Turn the banana inside out into a microwave safe bowl.
When using previously frozen bananas, it will take less time for the juices to release in STEP 1.
You'll need at least one fresh banana for garnishing the tops as shown in the photo.
Fold 1/2 cup chocolate chips, candied ginger, or dried fruit into the batter with the nuts or in place of the nuts.
Pecans, hazelnuts, or sliced almonds can be substituted for the walnuts.