Preheat the oven to 350 degrees F. Spray a 9x5 inch loaf pan with nonstick baking spray.
In a small bowl, whisk together the flour, baking soda, cinnamon and salt.
In a large bowl, whisk together the melted butter and brown sugar. Whisk in the eggs one at a time. Whisk in the vanilla extract.
Stir in the dry ingredients until just combined. Stir in the sour cream until combined. Stir in the zucchini until combined.
In a smaller bowl, stir together the sugar and cinnamon.
Pour half of the batter in the loaf pan. Sprinkle three fourths of the cinnamon sugar mixture over the batter. Top with the remaining half of the batter. Sprinkle on the cinnamon sugar and swirl it into the top.
Bake for 50 to 55 minutes, or until a tester inserted in the center comes out clean. Let cool completely before slicing!