In a light-colored saucepan over medium heat, melt 6 tablespoons unsalted butter, stirring frequently. Cook until the butter foams and then turns a nutty golden brown with specks. Remove from heat immediately and let cool slightly.

Preheat your oven to 350°F. Grease a 9x5-inch loaf pan and line it with parchment paper, leaving an overhang on the long sides for easy removal.

Wash and dry the zucchinis, then shred them to yield about 2 cups. Gently squeeze out the excess moisture using a clean kitchen towel.

In a large bowl, whisk together 1 3/4 cups all-purpose flour, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, and 1/2 teaspoon salt.

In another bowl, whisk together 1 cup granulated sugar, 1/2 cup packed brown sugar, and 2 large eggs until smooth. Stir in 1 teaspoon vanilla extract and the cooled brown butter.

Add the dry ingredients to the wet ingredients in two batches, folding gently with a spatula until just combined. Fold in the shredded zucchini and 1/2 cup chopped walnuts evenly.

In a small bowl, mix 1/4 cup brown sugar, 1/4 cup all-purpose flour, and 1/2 teaspoon ground cinnamon. Cut in 3 tablespoons cold cubed butter using your fingers or a pastry cutter until coarse crumbs form.

Pour the batter into the prepared loaf pan and smooth the top. Evenly sprinkle the walnut streusel over the batter.

Bake for 50-60 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs but no wet batter. If the top browns too quickly, tent loosely with foil after 40 minutes.

Let the bread cool in the pan for 15 minutes, then lift it out using the parchment overhang and cool completely on a wire rack before slicing.