In a small saucepan, heat butter over medium-low heat until the milk solids turn a rich, toasty brown color with a corresponding aroma. This should take about 15 minutes. Keep an eye on it to make sure it doesn’t burn.
Heat oven to 350 degrees. Grease either a 9 x 5 loaf pan or a 12-cup muffin tin, or use paper muffin liners.
In a medium bowl, whisk together both flours, salt, baking soda and cardamom.
In a large bowl, beat eggs, butter, vanilla and sugar until smooth. Stir in grated zucchini.
Add the dry ingredients to the wet and stir until just combined. Don’t overdo the mixing! Gently fold in the toasted walnuts.
For muffins: spoon into prepared muffin tin, filling each cup about three-quarters of the way full. Sprinkle the streusel topping on each one. Bake immediately for 20-25 minutes or until a toothpick inserted into one of the muffins comes out with just a few crumbs attached. Cool on a rack for 10 minutes, then turn out onto rack to finish cooling. Or just eat!
For loaf: Pour batter into prepared loaf pan and sprinkle streusel on top. Bake for 45 to 60 minutes, or until a toothpick inserted into the middle of the load comes out with just a few crumbs attached. Cool on a rack for 15 minutes, then turn out of pan onto the rack to finish cooling.