Preheat oven to 350°F. Spray 8½ x 4½-inch loaf pan with nonstick cooking spray, then line the bottom and sides with parchment paper (to make it easier to remove the baked loaf from the pan for cooling).

Defrost frozen, peeled bananas by placing in a microwave-safe bowl, covering bowl loosely with paper towel and defrosting in microwave oven for about 3 minutes, or until mostly thawed. Pour microwave-thawed bananas and liquid into a fine-mesh strainer over medium bowl and allow to thaw completely and drain on the countertop.

Whisk all-purpose flour, whole wheat flour, baking soda, and salt together in a large bowl.

Transfer banana liquid to medium saucepan and cook over medium heat until reduced to ¼ cup, about 5 minutes. Remove pan from heat, stir reduced liquid into bananas, and mash with until smooth. Whisk in butter, eggs, brown sugar, and vanilla.

Pour banana mixture into flour mixture and stir until just combined. (Cook's Illustrated: Fold in chopped walnuts if you're using them.)

Scrape batter into prepared pan. Slice remaining banana diagonally into ¼-inch-thick slices. Arrange banana slices on top, on either side of loaf.

Bake banana bread until toothpick inserted in center of loaf comes out clean. Baking time can take anywhere from 55 to 75 minutes depending on your oven; mine took 65 minutes.

Cool bread in pan on wire rack 15 minutes, then remove loaf from pan and continue to cool on wire rack.