Preheat oven to 350°F. Line the bottom of a loaf pan with parchment paper and spray loaf pan with nonstick cooking spray.
Place 5 bananas in microwave-safe bowl; cover with plastic wrap and fold one edge back to allow steam to release. Microwave on high until bananas have released liquid, about 5 minutes. Transfer bananas to a mesh strainer placed over bowl. Allow the bananas to drain, stirring occasionally.
While bananas are draining, combine flours, baking soda, and salt in a large bowl; melt butter; chop nuts.
Once bananas have released their liquid, transfer liquid to saucepan and cook over medium-high heat to reduce. You should end up with about ¼ cup. Put bananas in the bowl that held the drained liquid. Stir reduced liquid into bananas, and mash with potato masher until smooth. Whisk in butter, eggs, brown sugar, and vanilla until combined.
Pour banana mixture into flour mixture and stir only until the flour is moistened. Gently fold in walnuts and pour batter into prepared pan. Sprinkle raw sugar evenly over the top.
Bake until toothpick inserted in center of loaf comes out clean, 50 to 65 minutes. Begin checking loaf at 45 minutes—depending on the size of loaf pan, the cooking time will vary. Cool in the pan on wire rack 15 minutes before removing loaf. Continue to cool on the wire rack. Serve warm or at room temperature.
Notes
Makes One 9-inch loafThe bananas for this recipe need to be very ripe, even dark brown to produce a rich banana flavor.For 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.
For 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.