Preheat oven to 350°F (175°C). Butter a 9 x 5-inch loaf pan (or spray with nonstick cooking spray), then set aside.
Mix 6 tablespoons butter and ⅔ cup brown sugar until light and fluffy, using a stand mixer (or hand mixer).
Add 2 eggs and 1 teaspoon vanilla extract, then mix until fully incorporated.
Stir in 1 cup mashed banana by hand. Set aside.
In a separate bowl, whisk together 1 ½ cups flour, 1 teaspoon cinnamon, ¾ teaspoon salt, ½ teaspoon baking soda, and ¼ teaspoon baking powder.
Using a spatula, gently fold flour mixture into butter mixture, stopping when it's about halfway mixed in. (There should still be flour visible when you are done.)
Add 2 cups diced apple and ¾ cup chopped walnuts. Using spatula to fold them into the batter, stopping when everything is just combined (do not over-mix).
Transfer batter to loaf pan, then bake for about 45 to 55 minutes, or until a toothpick inserted in the center comes out mostly clean.
Let bread cool in pan for 15 minutes, then transfer to a wire rack to cool completely before adding glaze.
Prepare Glaze + Topping:
Add 1 cup powdered sugar, 3 tablespoons maple syrup, ½ teaspoon vanilla, and a pinch salt to a small bowl. Stir until totally smooth.
Use a spoon to drizzle the glaze over the cooled banana bread. (You may have a little left over.)
Sprinkle the finely chopped walnuts over the glazed loaf.
Serve bread immediately, or store in an airtight container until ready to eat.
Notes
Apples - Any crisp, tart (or tart-sweet) apple will work, such as Granny Smith, Honeycrisp, Pink Lady, or Cosmic Crisp. You can peel the apples if you prefer, or leave the skin on (it will soften as the apples cook).Bananas - For the best results, wait until your bananas have plenty of brown spots before making the bread. Perfectly ripe bananas won't work well.Measuring the Flour - Lightly spoon the flour into a dry measuring cup, then use a knife (or something else with a straight edge) to level off the top. Do not scoop the measuring cup into the flour. Scooping the flour packs it into the cup and is likely to make your bread dry.Making the Glaze - The Maple Glaze should be thick, but drizzle-able. If the glaze is too thick, you can thin it by adding more maple syrup (½ teaspoon at a time). If it's too thin, add more powder sugar (bit by bit) until you get the right consistency.Adding the Glaze - Save on cleanup time by placing a piece of foil or parchment paper under the wire rack (or set it on a baking sheet) before drizzling the glaze over the bread.More - Check out the info before this recipe card for helpful tips, step by step photos, recipe variations, and more!