Preheat oven to 375°F. Place the bananas, still in their peels, in a roasting pan or on a sheet tray. Bake until bananas are soft and browned, 12 to 15 minutes. Remove and let cool slightly, then scoop out the banana flesh and measure out 1 cup mashed banana.

In a medium mixing bowl, combine the flour, baking soda, salt, cinnamon, and crushed walnuts. In a separate mixing bowl, stir together mashed banana, melted butter, maple syrup, and vanilla extract. Whisk the eggs and add them to the maple syrup mixture. Pour the wet ingredients into the dry and stir until just combined.

Pour the batter into a lightly greased (or lined with parchment paper) standard (8-inch x 4-inch) bread pan. Bake bread until golden and a knife comes out clean when inserted into the bread, 50 to 60 minutes. Let cool for 5 minutes in the pan then transfer to a rack to finish cooling.