• Preheat the oven to 350℉ / 180℃. Grease the pan and line it with parchment paper with about 2 inches of overhang on each side. Set it aside.

Make the streusel topping

• Put the brown sugar, flour, ground cinnamon, salt and cold butter in a food processor. Pulse about 3-5 times until the mixture becomes crumbly with some big and some small pieces. Alternatively, you can use a fork, a pastry cutter or even your fingers to combine everything until you get big and small chunks of butter.
  ⅓ cup light brown sugar, packed (70g), ½ cup all purpose flour (62.5g), ¼ cup unsalted butter, cold and cut into cubes (57g), ½ teaspoon ground cinnamon, pinch of salt
• Spread the streusel on a plate and put it in the refrigerator.

Make the banana bread

• In a large mixing bowl, whisk together the flour, baking powder, baking soda, ground cinnamon, and salt. Set aside.
  2 cups all purpose flour (250g), 1 teaspoon baking powder, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, ¼ teaspoon salt
• In another bowl, whisk together the eggs, packed brown sugar, vegetable oil, melted butter, and vanilla until everything is well-combined.
  2 medium eggs, at room temperature, ¾ cup light brown sugar, packed (150g), 3 tablespoons light vegetable oil (45g), 3 tablespoons unsalted butter, melted and cooled (45g), 1 teaspoon vanilla
• Peel the bananas, weigh them (500g), and then mash them. I use a fork or potato masher (I like to keep some big chunks). Alternatively, you can mash them and then measure out 2 cups.
  2 cups ripe, mashed bananas (about 4 large bananas) (500g)
• Add the mashed banana to the egg mixture and fold it with a rubber spatula until just combined.
• The pour it into the flour mixture and add the chopped walnuts. Gently fold everything together with the spatula until just combined and you don't see any streaks or pockets of flour. Don't overmix it.
  ½ cup chopped walnuts (75g)
• Pour and spread the batter into the prepared pan.
• Take the streusel topping out of the refrigerator and sprinkle it all over the top of the batter.
• Bake for about 60-70 minutes, or until a toothpick inserted in the middle comes out clean. After about 30 minutes, cover the bread loosely with foil so that it doesn't get too brown. The bread is ready when a toothpick inserted in the middle comes out clean, with just a few tiny moist crumbs. Every oven is different, so start checking on the bread every 5 minutes, once you reach the 60-minute point.
• When done, take it out of the oven and let it cool in the pan for about 30 minutes.
• Use the overhanging parchment paper to lift the banana bread out of the pan and onto a wire rack to cool completely before slicing (about an hour).
• Enjoy!