Preheat the oven to 350F (175C). Take a 9" x 5" loaf pan, butter the sides and bottom. Place a piece of parchment paper along the long side of the pan such that the parchment paper goes up the sides and can be used as a handle to lift out the banana loaf after baking. Butter the parchment paper and flour the sides and bottom of the pan.

Create the crunch topping mixture. In a medium bowl, add in 1/2 cup of all-purpose flour, 1/4 cup of finely chopped walnuts, 1/4 packed cup of dark brown sugar, and 1 tsp ground cinnamon. Mix together till well combined. Then add 6 tbsp of room temperature unsalted butter, and mix with a fork or pastry cutter until the mixture is crumbly.

In a small mixing bowl, whisk together 2 medium-sized ripe bananas and 1/2 cup of sour cream until the mixture is smooth. Set this aside. Take a medium bowl and a sieve, add in 1 1/2 cups of all-purpose flour, 1 tsp of salt, 1/2 tsp of baking powder, and 1/4 tsp of baking soda. Sift these together, whisk to combine, and set aside.

In the bowl of your stand mixer fitted with a paddle attachment, add in 1/2 cup (1 stick) of room temperature unsalted butter. Mix till the butter is creamed and smooth. Stop the mixer and add in 1 packed cup of light brown sugar. Continue mixing until the butter and sugar are creamed together and look light and fluffy. Stop the mixer, scrape down the bowl, and continue mixing for another 20 seconds.

With the mixer on medium speed, add in 4 room temperature eggs, one egg at a time, making sure the previous egg is mixed in before adding the next one. Once all the eggs are added, stop the mixer and scrape down the bowl.

Add in the vanilla and mix until incorporated.

With the mixer on low, add the flour mixture alternating with the banana mixture, just until incorporated. Scrape the bowl down and use your spatula to mix in any pockets of flour or butter that remain.

Transfer this batter into the prepared loaf pan then use a spatula to smooth out the surface as much as possible. Using your hands, sprinkle the coarse, pea-sized crumbs of the crunch topping evenly over the batter.

Bake at 350F (175C) for about 50 to 55 minutes or until the center is set and a toothpick comes out clean.

Let the finished loaf cool in the pan for 10 minutes. After 10 minutes, use a sharp knife to unstick the sides and corners without parchment paper as needed. Then use the parchment paper handles to lift the loaf out of the pan and onto a cooling rack. Let it continue cooling till desired temperature. Enjoy warm or at room temperature!