Preheat the oven to 350°F (180°C) and grease a 9x5-inch loaf pan. Line the pan with a strip of parchment paper leaving an overhang to easily lift out the bread.
Combine flour, baking soda, salt, and cinnamon. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar together for about 4 minutes until light and fluffy.
Add eggs, one at a time, mixing 20 seconds after each addition. Then add vanilla extract, yogurt, and mashed bananas. Mix until incorporated.
Transfer the flour mixture to the wet ingredients and combine until just incorporated and no flour pockets remain. Fold in the nuts and transfer the batter to the prepared pan.
To make the topping, melt butter over medium heat. Stir in maple syrup and whisk to combine. Add chopped walnuts and cook for 1-2 minutes, stirring constantly, until the mixture thickens. Distribute the topping evenly on top of the batter.
Bake for 55-65 minutes or until a toothpick inserted into the center of the bread comes out clean. Cover the pan with aluminum foil after 30 minutes.
Let the bread cool in the pan on a cooling rack for 5-10 minutes. Then lift the bread out of the pan and let cool completely on the rack.